Online forums that exist expressly for the purpose of posting images and/or disscussions. They usually have a variety of sub-boards, each with its own focus. Unlike most forums, threads are never archived by the site, so if nobody posts in one for a while, it will eventually be deleted.
The imageboard as it's usually thought of today began in Japan. Futaba Channel, a spin-off of the popular text-based discussion board 2-Channelnote Officially pronounced "Ni-Channel", but most English netizens are comfortable with just calling it "Two-Channel" (2ch) is probably the best-known. Sometimes the blanket term 'wakaba style' board is used (wakaba referring to the board software that runs the board), traditionally allowing anonymous posting with a self-enforcing theme and standard that hopefully keeps it from becoming too mediocre.
The best-known in the US is probably 4chan, created in 2003 to serve as an English-language version of Futaba Channel. Especially among Americanotakunote Or, although the term is used more in a negative sense, "Weeaboo", it's quite popular for the generator of memes and, due to anonymous users and loose moderation, extremely variable quality; for these reasons, one could see imageboards to be the antithesis of social networking sites such as Twitter and Facebook.
Most American wakaba boards trace their heritage to 4chan, as alternate boards proliferated when 4chan went offline in its formative years. The successful ones tend to cover more specific niches, like meganekko or sad girls in snow. Or guro hentai. Or lolicon. Or guro hentai with underage meganekkos in snow. Or guro hentai with underage meganekkos in snow weeping profusely over image boards. Or... you know what, let's just stop talking about it.
The only real standing rules of just about every imageboard site in existence (besides specific topic rules for subforums) is that its users can't post anything considered illegal in a site's area of hosting (many admins turn a blind eye to things like unauthorized filesharing, however). Dodgy to outright highly illegal material is occasionally posted anyway just for the pleasure of subverting this rule; mods unsurprisingly counter this with anything from warnings to reports to the FBI.
What many imageboard sites have in common is using the same distinctive layout and interface from 2ch: there is a sidebar either on the left, bottom, or top (but never the right) that lists each board on the site in a simple URL theme (usually a contraction of their topic name), and to the right of this is the actual site content. Most boards are also set up fairly similarly to 2ch, (/v/, /m/, /tg/, etc) so you can usually count on wholly different sites to have the same general setup.
Not to be confused with our own Image Links Wiki, or Image Booru.
Notable Imageboards (every link below should be considered NSFW unless specifically mentioned otherwise):
While not an imageboard itself, The Overchan V.2 maintained an (as up-to-date as humanly possible) list of every English-language imageboard in existence (including furry imageboards and text-based 2ch-esque boards). It was also the closest thing to a worksafe link from this point forward.
The aforementioned Futaba Channel. Take note that unless you live in Japan, you can't post there. This is to prevent crapflooding and DDoSing from foreigners outside of Japan. And even if you could, it is much more chaotic than 4chan. Most of the imageboards frequently have discussions and images that are unrelated to its intended purpose and spam is basically omnipresent. It has a lot more boards than 4chan and churns out memes at an even faster rate.
The also-aforementioned 4chan, which hosts boards dedicated to just about every generalized subject you can think of (and a few specialized subjects, as well), is home to the self-proclaimed "final boss of the Internet" (the Random [Bullshit] board, aka /b/), and is the starting point for about half the memes in the various subpages of the Memetic Mutation article. Humorously enough, 4chan was created by a bunch of regulars from Something Awful's anime subforum, a fact that is best left unmentioned on both sites. Has its infamous, luckless spin-off, 7chan, Which has its own spin-off, 99chan (see below). The users consider them 'better' than 7chan - and sometimes even 4chan.
/a/ (Anime and Manga) is basically the same as /v/ (see below), but with anime, and "live action" television shows that are distinctly rooted in, or based off of an anime or manga series. Mostly discusses the more popular series, and upcoming shows they think will become popular. New anime gets released at a much faster rate than American cartoons, so there's usually plenty to discuss.
/adv/ (advice) is for advice, mostly relationship, and about as good as you can expect from a random group of completely anonymous people on the internet.
/an/ (animals and nature) is mainly full of topics about animals, both wild and domesticated. Topics like "What's your favorite dinosaur" or "How do I take care of my pet" are common. Animal cruelty is not allowed though, and you will be banned if you post it.
/asp/ (alternative sports) is for discussion of sports that aren't major, professional ones.
/b/ (random) is not all of 4chan, everyone just thinks it is. There are no real rules in /b/, so people are free to post whatever non-criminal material they want. In practice, that means a lot of porn, pictures, and discussions that can only be defined as, well, random. Notably, it can have, and undoubtedly has had, every single one of the other boards' topics discussed on it, with the main difference being more chanspeak. Another type of popular threads involve "posts ending in x will decide y", referring to the post number, which are treated with mixed reception. /b/ is also the source of what might be called chanspeak, which is a dialect of net slang that is best described as mixture ofAcronym and Abbreviation Overload and Cluster F-Bomb. The word "fag" is particularly common, to the point where if you actually wanted to call someone a f... cigarette, you'd have to use "gayfag" or "fagfag".)
/c/ (cute anime), and /cm/ (cute men) are work safe boards for cute animated things. /cm is for cute men (animated only), and /c is for everything else that's both animated and cute including girls, couples, and animals. It has quite a few chibis because of that.
/cgl/ (cosplay and egl) (one board, egl is part of the title) where people show off the cosplay they work on, talk about conventions they went to and what they cosplayed as, and general cosplay tips. They're the place to go if you need help on a cosplay of your own. Be warned, however, that on off days /cgl/ is one of the most vicious hotbeds of cosplay drama on the entire internet.
/ck/ (food and cooking) is usually filled with a mix of delicious recipes, people asking for delicious recipes, and general cooking tips. Speaking of tips, never, under any circumstances, browse /ck/ on an empty stomach.
/co/ (Western Animation and Comic Books) is the Western version of /a/, for all non-asian comics and cartoons. /co/ can get kind of obsessive about shows and girls it really likes. When a new show it likes starts airing, the board usually becomes flooded with topics about it, which eventually resulted in the rule that only one My Little Pony thread could be active at a time, after people complained.
Apparently the one-thread policy didn't seem to be work as far as My Little Pony is concerned, as individual pony posts and threads would still appear all over the boards, drawing the irk of various 4chan-goers. During the February 2012 reorganization of the boards, one anonymous poster suggested that an MLP board be created to confine pony-related posts, but it wasn't until a plea from an MLP fan against it implied such a board would ruin the fandom (i.e. at /vp/) that the admin made up his mind. Thus /mlp/ (Pony) (see below) came to be.
/diy/ (Do-It-Yourself) is for do-it-yourself projects, whether it's fixing something broken or creating something from scratch.
/e/ (ecchi). is the softcore porn board, separating it from /h/, which is for hardcore images. Ecchi material is suggestive and often times cute, but it is still an adult board with adult material that does allow anatomically-correct nudity.
/fit/ (fitness) is a board dedicated to healthy living and exercise routines, although the main focus seems to be on strength training. Posters will frequently joke on how obsessive its members are with the littlest of details in fitness routines, squats and oats, maintaining "gains," and despite all of their efforts, still cannot get a girlfriend. If you manage to get pass the broscience and trolling, a few good fitness tips can be found here, especially in Harsh's Worksheet.. No homo.
/g/ (Technolo/g/y) is dedicated to mainstream tech discussion. Widely dominated by desktop computers and audio players trollfests (though there are ocassions where an exceedingly good thread comes out, for example, the sat-comms Q&A), this board is known for the insane amount of Windows vs. Mac vs. Linux threads, the endless desktop/battlestation/guts thread, the insane amount of trolling and newbie baiting (deleting system32 and installing Gentoo as the most notorious examples), nVidia vs. ATI/Intel vs. AMD/Android vs. iOS, C# vs. Java vs. Python (actually every tech rivalry known to man will have at least one thread per month), hating on audiophiles, hating on Arch Linux, arguing about anything, and the occasional genuine help thread. Its main contributions are the Stallman copypastas, the 1.7 meme, and ITS OVER, _____ IS FINISHED. On a more positive note, the daily programming threads and the occasional tech community thread (mostly discussing jobs in the tech industry) are what saves this board from extinction. Also, the most excellent Falcon's guide, a guide that can help you with planning what to buy for your next computer, has its origins here.
/gd/ (graphic design) is for discussion of graphic design software and computer-aided graphic design techniques (i.e, 'photoshopping').
/gif/, a board for posting looping animated images, or GIFs. Since the creation of /wsg/ below, however, it's only really ever used for porn.
/hc/ (Hardcore) is dedicated to hardcore pornography.
/hm/ (Handsome Men) is for real-life pictures of men, be it safe or pornographic (all of which is of a homosexual nature).
/hr/ (high-resolution) is for very large images in general. Often a good place to look for wallpapers, though this board can and often does veer on the NSFW side of things.
And then there's /jp/ (otaku culture). Its conception was an ill-defined split in the user base and topic control of the original "/a/ - Anime and Manga" stemming from the bulk output of Fate/Stay Night and Tsukihime and the Touhou series upsetting other /a/ content. Outsiders might comment that /jp/ doesn't have any real direction and patrons might comment back "Actually, we're not very fond of Japan."
/k/ (weapons) is about weapons. While firearms tend to be the most talked about, swords and other melee weapons, armor, and all kinds of military hardware and equipment are also covered here. Topics about hunting and wilderness survival pop up occasionally as well. While they generally don't like video games, STALKER threads tend to get a free pass.
/lgbt/ (Lesbian, Gay, Bisexual, & Transgender) is for the respectful, work safe discussion of LGBT lifestyles and the LGBT community.
/lit/ (literature) is for discussing professional stories. That means fanfics are not allowed. Other than that, the board talks mainly about their favorite books, and is a great place to go if you want a recommendation for something to read. Be warned; at their worst, they can be the /v/ of literature, although they are on topic.
/mu/ (Music) has discussions about all genres of music (especially the ones you haven't heard of). The /mu/tants are not very welcoming, as they are obsessed with arguing or trolling over which music is "patrician" and which music is "plebeian". They are also obsessed with music review site Pitchfork and Italian expert Piero Scaruffi. Some of the most commented artists in /mu/ are Neutral Milk Hotel, Animal Collective, Radiohead, Merzbow. Occasionaly, there are "share threads", in which people would post download links to various albums.
There's also /n/ (transportation), which is a very slow (for 4chan) board that's generally 95% bikes, 2% trains, 2% planes, and 1% people lost on the way to...
/out/ (outdoors) is for discussions about hiking, camping, geocaching, orienteering, gardening, etc, and other traditional ways of enjoying the great outdoors.
/p/, the Photography board, Containing an almost paradoxical hatred for Hipsters and post-process while simultaneously gushing over vintage camera equipment and photoshop gurus, /p/ has the distinction of being possibly the single most concentrated collection of image retouching talent on the entire internet. When they aren't showing off their retouching ability (or mocking each other by photoshopping realistic genitalia into one another's photographs,) /p/ tends to entertain itself by way of vast galloping flame-wars over camera equipment and photographic styles. Just, for the love of whichever god you worship, if you post your images to /p/ remember to resize them to 1,000 pixels on the longest side. You really don't want to see what happens if you don't.
/po/ (papercraft and origami) isn't very populated, but they're generally pretty helpful to anyone who shares their interests. Full of people showing off their awesome works of paper and stencils.
/pol/ (Politically Incorrect) is the board for politics, current events, and general debate. Much broader in scope than the board it supposedly revived, /n/ (News), which was dedicated to news only. Revived due to user demand now that election season is underway. Flamewar central, with many extreme opinions that would earn bans on other forums duking it out. Is also filled with threads debating which race has the most attractive women. Ron Paul is perhaps the most beloved of American politicians by /pol/.
/q/ (4chan Discussion) is for discussing issues pertaining to the site itself, such as rules clarification, site features suggestions, and others misc. 4chan-related topics.
/r/ (Request) is where it is possible to request where to find each and every one of the above topics. Much porn hunt abounds. Infamous for the great majority of requests going unanswered.
/r9k/ (Robot 9001) is a board based off Randall Munroe'sROBOT9000 idea. In practice, it's kind of like /b/, but with less porn and dead memes. Tends to have (relatively) intelligent discussions about anything and everything. A good deal of the time the board is filled with misogyny and ronreyness. The few remaining meetup threads and rate me threads belong in /soc/.
/s4s/ (Shit 4chan Says) was originally an April Fool's prank meant to mimic the subreddit "Shit Reddit Says" but do to it being NSFW as well as including things long gone from /b/ (No Forced Anon, dubs) it has now become a permanent edition. /s4s/'s main call is dubs (when a post number ends in repeating digits) and stealing other board's gets (Major posts like a board's 100,000,000 post or post 34567890).
/sci/ (science and math) is for people interested in science and the latest scientific advances, or needing help understanding a math problem they don't get or a science project they just can't make work. Also home to a great many science vs. religion arguments (surprise surprise).
/sp/ (sports) takes the already heated topic of sports fandom and adds that special 4chan touch. /sp/ hates all teams in general and your team specifically, though local fans will often show up to defend their team when the arguments get really heated. The big draws are game threads following and commenting on the action live (good games for the major sports can require multiple threads that hit the limit, and major playoff games are guaranteed to go something like 7-10 threads long). Thanks to the large number of users all over the world, /sp/ is one of the few boards to have distinct variations. There is the Soccer and Lad banter filled "yuro /sp/" and the Football/Baseball/Basketball and various other sports filled "murikan /sp/. While the two sides tend not to agree on a lot of things both are well known posting lots of Power Rankings threads, Feel Threads, Mila Kunis threads, Ian Terry threads and plenty of Tony Kornheiser which has caused many anons to rename /sp/ to the "Shit Posting" board.
/tv/ (Television & Film) It's /mu/, but with movies. And celebrities. Lots of them. It's one of the few places where you can freely post pics of women feet or compliment child actresses in unambiguous way without being called a creep. /tv/ tends to be obsessed with certain shows, like Breaking Bad, Game of Thrones and, back in the day, LOST. Generally, /tv/ is a good place to discuss new episodes of various shows, if you can stomach waifu stuff and occasional /b/ tier comments. However, with movies, /tv/ may be very picky, and it will most certainly hate vast majority of the new ones (go ahead, try talking about The Dark Knight Rises). Wrestling and some anime movies go also go here. Like /fit/ and /r9k/, /tv/ will remind you daily tfw no gf.
/u/ and /y/, the Yuri and Yaoi boards. Both are known for their tendency to see homosexual relationships where none exist. Of course if they didn't, they wouldn't have all that much to talk about. This means they talk a lot about shows like K-ON or Kim Possible, which have no explicit (or really even implicit) homosexuality, but a fan base that really likes to pretend they do, as well as more explicit cases of homosexuality. Neither one moves very fast, so it's not uncommon to see a thread linking to 4 or 5 previous ones with the same topic that no longer allow images.
One sore point of discussion on /v/ is how gaming journalism is pro-corporate rather that pro-consumer, especially when controversial news of paid reviews, review embargos, and publisher blacklisting are told with increasing regularity. Tired of the crap from the current gaming media, several /v/-goers formed Gather Your Party.
Several members of /v/ had created the wiki site "/v/'s Recommended Games" as a response to many threads asking what to play next. The community there is a lot nicer too.
/vg/ (Video Game Generals) is a "spinoff" board of /v/. It's meant to offload the "general" threads clogging /v/: long-term oriented, recurring threads focusing on a specific video game that get recreated as soon as the old thread dies.
Another board borne of /v/ overload is /vp/ (Pokémon). Pokémon discussion clogged up the /v/ board so badly, they made another board, just for Pokémon. That should tell you how popular Pokémon is there.
Yet another spin-off of /v/ from March 2013 is /vr/ (retro games), for the discussion of classic, or 'retro' games. Retro gaming means consoles, computer games, arcade games (including pinball) and any other forms of video games circa 1999 and earlier, but sixth generation is not considered retro, thus rendering the [[Dreamcast]] "not retro". Previously in a one-month tryout phase, it's meant to offer discussion of retro games without the flamewars and bickering of /v/. It is now a permanent board.
/w/ (anime wallpapers) is for very large pictures related to animes, to be used as wallpapers on computers. They don't like people requesting Wallpapers.
/wg/ (wallpaper general) is for very large pictures not related to animes, to be used as wallpapers on computers.
/wsg/ (Worksafe GIF) is for... well, worksafe .gifs. Great for reaction images. Being 4chan though, probably should not be viewed at work, as moderately NSFW images will still be present.
/x/, (Paranormal; as in the X-Files) is the creepy, scary, paranormal, and general "weird shit" board. /x/ is the origin of a great deal of the "creepypasta" floating around the Internet, which eventually led to the creation of SCP Foundation.
/3/ (3dcg) is for 3d modeling, sculpting, and so on. By far the slowest board on the site, there are often threads that have been active for nearly a year. Relatively docile and calm compared to the other boards. Doesn't like porn or people who ask how to make porn.
420chan, created as a result of one of 4chan's longer downtimes, is a site specifically geared towards drug users (with several boards dedicated to different types of controlled substances; the site refuses to allow people to directly buy or sell drugs through the site itself, though). It also has several non-drug-related boards, including the site's most second-most-visited board, /wooo/. /wooo/ is one of the most well-known Professional Wrestling imageboards on the 'Net, to the point where Wrestlecrap considers the board their Arch-Enemy and forbids their Message Board users to mention it. (420chan's most visited board is, as you might expect, /weed/.)
More recently, posting on the Netjester board has become a popular pastime.
Fchan, which is the most well-known furry (primarily erotic furry) imageboard out there; although assumed to be created as a response to 4chan's infamous "April Fur's Day" prank, it was actually in existence for a while prior to it (and was created as a result of 5chan — which hosted a couple of furry-based boards — dying). The site set something of a standard for future furry imageboards by segregating content into different boards (one for female-only art, one for male-only art, and so on); it also marked the beginning of the term "DNP" ("Do Not Post") in the Furry Fandom, due to the list of artists who asked that their work not be posted on the site. Fchan's heavyhanded moderation and large DNP list led to several furry imageboards coming into existence to counteract Fchan's perceived stupidity, the most notable of which are:
Lulz.net, which is sometimes referred to just as /furi/ after the site's main board; it is pretty much always EXTREMELYNSFW, and not always because of furry porn — and not always because of porn. Many of the board's threads are about gore pictures, grossout images, trolling, and extremely vitriolic politics. There's also more furry-hate on the site than /b/ and Something Awful combined. Even the board's regulars will often admit that the site is basically "/b/, but with less furry porn".
If you go in without Adblock prepared beforehand, it's also a great stress test for your antivirus application of choice.
That 5chan may have died, but there's now a totally unrelated new 5chan which adds a post voting (community moderation) feature modelled after Reddit. Except it's currently down.
55chan, the Brazilian imageboard. The site used to attract a good number of functionally illiterate users, commonly assumed coming from the social networking site Orkut. Those users would usually flood the boards with bland topics and requesting the other users to raid their enemies' pages on Orkut. The moderation eventually put a stop to this by enabling a filter that asked the user to copy a simple sentence using capitals and punctuation without using the copy-paste command; only once this had happened would a user be allowed to use the boards. That filter was enough to fend off the vast majority of the problem users.
Two other Brazilian imageboards spawned when 55chan was down: BRchan and 138chan. If 55chan was the Brazilian 4chan, it is safe to say that BRchan and 138chan are, respectively, 7chan and 99chan.
888chan was the imageboard chiefly used by Project Chanology, an Internet-based movement based on the Anonymous "culture" borne from 4chan's /b/ (the group has no defined leaders, and most of the members of the various Chanology subgroups do their best to remain anonymous) which was (and still is) dedicated to "fighting" the Church of Scientology. (Project Chanology itself has been going for more than two years as of this writing; it has used several different imageboards as their base, with 4chan's /b/ being the original starting point of the movement.)
888Chan, as of July 2011, has been dead for nearly two years; things have moved over to Vortex Chan, including the pseudo-intellectual concept that would be considered Serious Business to the average *chan user.
As of 2013, 888chan now has a board for lolcow discussion and harbors the remnants of 789chan's users
Because of the area's oddly high concentration of /b/tards, someone in the Kansas City (oh, that's why) area made the now defunct 913chan, naming it after the KC area code.
Desuchan, the result of a certainmeme becoming too big and obnoxious for the other imageboards to stand. ~desu (pretty much worksafe)
It also has its fair share of spinoff, mostly as backups during downtimes. Because of the current downtime, it should be noted that Hugachan is the current refuge for the Desuchan regulars.
Krautchan, pretty much THE German imageboard. Became famous in the wake of the Winneden school shooting, when the media fell for an extremely obvious fake post announcing it (a real but completely unrelated post photoshopped). Has an /int/ board. Its humour consists mostly of intentionally bad and literal translations of english memes and imageboard slang. (Beweis mich falsch, fappieren)
Nearly every European country (and a few non-European ones) has their own imageboard. Such as turulchan, the (new) Hungarian imageboard, Diochan for Italy, Kuvalauta (now Ylilauta) for Finland, Dejimachan for the Netherlands, SectaChan for Spain, severalRussianimageboards, and quite a few more (complete prospect◊). The /int/ board on Krautchan (where each user gets marked with his country's flag) works as a sort of common meeting ground.
Another British Image Board is B3TA, which in addition to message boards, also has weekly or so Image Challenges, quizzes, movies and games. While the vast majority of the userbase are okay folks and the image challenges bring out funny images, the site is NSFW, and don't forget to bring your brain bleach.
Actually, posting anything blatantly NSFW without a warning and a text only link usually tends to generate a fair bit of rage.
plus4chan, originally started as an offshoot of 4chan prime's /co/ (Comics and Cartoons) board, has since grown into its own standalone site with a very tight-knit userbase. Much of the content is still /co/ related, but there are also boards about anime, video games, cooking, music and television, and Homestuck, among others.
99chan. Elitist as hell, and damn proud of it. 99chan is even slower than 420chan, but has a huge variety of boards. Expect to be banned if you post demotivators, memes, or if the many mods are feeling bored. Most of the humor on the site revolves around inside jokes and self-deprecation. Practically everything is decided on the IRC (better known as the "circlejerk"), which is self-admittedly populated entirely with pseudo-intellectual gay Canadian alcoholics. Tends to have a "sekrit club" mentality, so if your post smells even faintly of 4chan, expect to be banned.
MikuChan is an image board dedicated to Hatsune Miku and other VOCALOID characters/voice banks
789chan, an image board with a focus on technology. It is, however, most well-known for its /cwc/ section, which, of course, focuses on Christian Chandler of Sonichu fame. Despite the title, very few threads are about Chris, and generally discusses several other people on the internet that users don't like. Was shut down on November 2012
Choroy Pride is a Chilean imageboard. Even with the NSFW material that pops frequently, it's pretty lighthearted compared with most of the imageboards it tries to emulate. With a permanent hatred of all the people that commit spelling errors (they have an explicit rule against it, and if you post with any spelling errors you will get banned), it is the most active imageboard from that country to date. It was made by some users unhappy with 6-chan (another Chilean imageboard) when that had legal issues. Most of the boards are locked for people that don't have a Chilean IP address, but they have an /intl/ (international) board that is open for outsiders.
Shrekchan a strange and somewhat active board with a focus on all things Shrek. Mostly full of shrek-themed images and NSFW greentext stories with a... unique version of chanspeak. Yes, it's ironic... Probably.