The citizens band craze of the 1970s is more or less gone, and although small handheld walkie-talkies (49MHz for kids, FRS/GMRS and PMR446 for grownups) are still popular, and two-way radios are used everywhere in job settings, the vast majority of people who use two-way radio only know it as the way their mobile phones function. This is a guide to how two-way radio works in the real world, so you don't get confused by Hollywood CB.
First off, the big difference between mobile phones and what most people think of as two-way radio is the fact that on a cell phone, both parties can speak at once over an open line (duplex communications). This is not traditionally so; on any given walkie-talkie or two-way mic, there's usually a push-to-talk button, and as a general rule you can't listen while you're transmitting (simplex communications). Two-way radios can be point to point (between two walkie-talkies for example) or use a repeater (transmit to base station, retransmit to whoever's listening); you can also have trunked communications, where the repeater has a system akin to a telephone PBX running behind it that groups communications into virtual channels regardless of frequency. The frequency has a significant impact on how the signal is propagated; as the frequency increases, the energy in the signal also increases, meaning that a channel near the AM broadcast band will behave very differently from a shortwave or FM broadcast (i.e. VHF) channel will. (In fact, the reason for the choice of shortwave — defined as 3MHz to 30MHz — as a band for international broadcast radio is because that particular set of frequencies hits a sweet spot for long propagation but inability to penetrate the ionosphere under certain conditions, so the signal bounces back to earth instead of being lost to space like FM or TV broadcasts.) Technically, two-way radio can be any transmission mode — voice, video, digital, or Morse code — but this article focuses mainly on analog voice.
A thing noted in the Hollywood CB article is that in many situations, a fictional character will pick up a mic and always be on the right frequency. This is not entirely false, but it only works if the party you're trying to contact is within range and listening to a calling frequency; these are specific channels that users of the band will tend to listen to as a sort of home frequency. (Some examples include Channel 19 for U.S. CB, 2182 KHz for maritime distress signals, and 14.285 MHz for upper sideband voice on the 20m amateur radio band.) However, when John McClane tries to contact law enforcement on CB in the Die Hard movies, that usually won't work; unless a local law enforcement agency is monitoring the citizens bands (probably emergency channel 9, and likely only for highway patrol purposes if that), the only other people to hear would be CB users. Most regulatory agencies assign specific frequency bands for public, broadcast, business, and public safety use, alongside things like ISM (industrial/scientific/medical) bands, that tend to get cluttered mainly with computer and cordless phone signals, among other utility signals, and although you can usually listen in with a scanner, you won't be able to transmit without modifying amateur gear or building your own, and that can get you in a lot of trouble. These allocations will frequently vary from country to country as well; for example, because of extensive use of unlicensed GMRS walkie-talkies by US civilians in border areas (where it was technically illegal by international treaty), Canada was forced to reassign a number of frequencies used there mainly for fire and other public safety use to the public because there was no way to control the flood of radio traffic. In addition, tourists from Europe coming to North America with PMR446 walkie-talkies frequently wind up stomping on US amateur radio bands that use the same allocation, and the US military got enough mileage out of FRS gear (often sent in care packages from home) that the Pentagon actually started ordering walkie-talkies keyed specifically to military channels.
In practice, real CB is quite a bit weirder than even shown above — in the US at least, most CB traffic is on shortwave in the 26MHz range, which means it's subject to some very strange atmospheric effects. You're actually prohibited by law from contacting stations over 150mi, which actually happens when conditions in the ionosphere are just right to reflect signals back to earth; since code enforcement on the US CB bands is all but nonexistent, "shooting skip" is actually somewhat common. In addition, quite a few CB users run amplifiers powerful enough to rival small commercial radio stations (which is quite illegal but in practice is only enforced on the retail level by the FCC), as well as special effects like echo mics. (Somewhere in the pre-Youtube days, someone put up a video of someone keying down a CB with a 70KW amplifier on it; the EMP emitted was so strong that it crashed the camcorder mid-shoot.)
Then there's amateur radio, aka ham radio. (Not "HAM" radio. It's not an acronym.) Once upon a time, before computers, it was one of the major geek preoccupations along with pulp fiction and the like; these days, it's seen as being largely the preserve of grouchy old men who are too far behind the times to use internet chat like everyone else. However, if you want to do anything experimental with radio, being a ham (and frequently finding an "elmer", or experienced mentor) is all but a necessity. Ham radio still has the advantage of being usable in parts of the world that are extremely isolated and sometimes even uninhabited; some of the stranger extreme adventure trips people take are for the sole purpose of going to some remote island like Bouvet or Clipperton and work (make contacts with) as many landside hams as they can reach, just so they can say they've been there. This also has the side effect that in years past, hams have provided critical communication services in disaster areas; although the need for this has dropped somewhat with the wide availability of satellite and cellular communications, the amateur community still comes in handy at times, and many national radio associations organize a yearly holiday of sorts called Field Day, which is largely devoted to practicing emergency communications in a contest format. (Of course, there's no shortage of people who take the hobby waaaaay too seriously, especially oldtimers who resent newbies who don't have to learn Morse Code, as well as people who put enough antennas on their cars to make them look like giant porcupines, but that's just like any other geek culture.) As for shooting skip, CB-style... not only do amateur radio operators have their own satellites to work from, but there's a few ambitious hams who regularly use the moon as a reflector for transmitting signals to people on Earth.
FRS and the other UHF-band personal radio allocations are a much different story; they're used much like police, fire, and business radios to maintain communications between a specific group of people. (These are the radios that advertise ridiculously long ranges, from 15 miles and up, that only really apply if you're transmitting from a balloon or the side of a mountain or something like that; UHF band signals transmit only along the line of sight and are easily blocked by surrounding objects.) These are mostly used for trips with family and/or friends as well as some small business uses (although this is usually a bad idea; better to get a license for specific business channels if security is an issue). Some of these (particularly the US General Mobile Radio Service) technically require licenses, although especially in the latter case, there are millions of compatible radios and most people don't bother to license, a fact that annoys license holders to no end. Chitchat with random strangers is not always welcome, especially as these radios are popular with kids who've outgrown the inexpensive 49MHz walkie-talkies with the Morse code buttons.
There are two uses for two-way radio that transcend national licensing though; the aviation and marine channels are more or less the same throughout the world. Aviation radio is unusual, as the band is just a bit higher than the FM broadcast band, but it uses AM broadcasting, since FM receivers only play the strongest signal, but AM receivers will play any signal that comes in, which is important for the high-information world of air traffic control. Marine radio, on the other hand, is split into VHF (for nearby boat-to-boat use as well as port operation use) and HF (i.e. shortwave), for worldwide communication, including distress signals. Marine VHF radios in particular are relatively cheap (if not as cheap as personal walkie-talkies), but you do not want to get caught transmitting on an in-use channel; at best, you'll be politely told to leave the channel, and at worst you might get slapped with a serious fine (or even arrested) for interfering with work or rescue communications. In both cases, the gear is more or less identical to other two-way radios, but the operating requirements are very, very different and a lot stricter, with specific channels and code terms ("prowords" or "prosigns" are the official terms). The lingua franca of the radio world is English; you aren't required to use it (unless you're a pilot; world aviation almost always requires pilots and air traffic controllers to speak English on the radio), but it's probably wise to start any conversation (at least on the amateur bands) in English unless you've arranged otherwise.
This leads into how using a two-way radio is different from a face-to-face conversation, or even a telephone call. Most uses of two-way radio place a high priority on unambiguous communication, leading to the creation of special vocabulary and codes, of which the most famous are probably the 10-codes that originated in police work in the United States and are widely used by CBers. (Ironically, the 10-codes are dying out among police, at least in part because only a few of the codes (like 10-4 (received) and 10-20 (location)) are universally understood between departments.) Amateur radio operators use a vocabulary of initialisms, many of which date from the early days of the Marconi Corporation, or even back to the earliest uses of landline Morse code; these include CQ (roughly, "Anyone out there?"), OM/YL (Old Man/Young Lady; "wife" is often translated as XYL), SK ("silent key" — usually means "I have stopped transmitting", but it's also used as a term of respect for the dead), and the Q codes, a series of three-letter codes originated in pre-WWI Britain and used primarily for operational and frequency issues. Since letters can be confused over a bad connection, it's very common to use a Military Alphabet, usually the NATO spelling alphabet, to spell things out over voice links. For identification, nearly everyone uses some kind of call sign; various countries have official callsign prefixes assigned by the International Telecommunications Union (the US uses A, N, W, K; the UK uses M,note , G, 2, V, and Z; Canada uses C and also some V signs inherited from the UK; Russia uses R and U; and so on), while CBers traditionally use pseudonyms or "handles". note
Finally, a word about gear. Radio was the first major application for semiconductor technology, way back when computers were analog devices and/or plans for complicated mechanical devices that never quite got built; the original cat's-whisker diode, created by touching a pin or thin wire to a chunk of galena (lead sulfide) or occasionally even the rust on an old razor blade, has long since given way to a silicon or germanium signal diode; in both cases, the diode forces the electricity from the signal to go through in only one direction, in the process removing the carrier wave (the signal itself) and leaving only the sound wave behind. The vast majority of radio gear over the last fifty years has been transistor-based, often with more or less computerized front ends. However, there's still a strong contingent of fans of vacuum tube radios, and they do still have their uses. In story terms, tube devices are usually simpler to construct (not to mention wonkier — tubes burn out just like light bulbs), but can be more robust in certain circumstances, such as stories After the End. (For example, John Lithgow's character at the end of The Day After, using an ancient tube-based ham radio to try to reach anyone who hasn't been blown up; in this case, the nuclear attack would have cooked any unhardened transistor-based devices, so the tube radio was pretty much it.) Also, as mentioned above, all radio has distance limitations based on power, frequency, and location of the transmitter, and just because you can hear them does not necessarily mean they can hear you, and vice versa.
There is a lot more about two-way radio and radio in general, but it gets obscenely technical and this article is long enough. The Other Wiki has immense amounts of information on radio technology, and if you're interested in learning about it, a good book on electronics plus something like the ARRL Handbook (from the American Radio Relay League) or the RSGB Communications Handbook (from the Radio Society of Great Britain) will tell you far more than can possibly fit (or belong) on TV Tropes. Radio slang and lore can be found in many places on the net — start with looking up "CB slang", as well as amateur radio's rettysnitch and wouff-hong.