Steam is a Digital Distribution platform for PC games, game soundtracks, and movies. Created by Valve in 2004 to distribute and integrate their own games cheaply, it has grown from its shaky and buggy beginnings into the service that a majority of PC gamers use for their online middleware needs (provided a user possesses a decently fast internet connection and a way of buying online). It also features automatic game updates, a messaging system, an online multiplayer platform, Achievements, and an in-game web browser, making it something like the PC equivalent of Xbox Live Arcade, except free and minus the ads. And like Xbox Live, it has since gone cross-platform, supporting Mac OS X and Linux in addition to Windows.
All of Valve's PC games since Half-Life 2 have required Steam, or rather the "Valve Anti-Cheat" (VAC) service embedded within it, and the retail versions of those games come with Steam bundled innote . The service is free to download as well, and allows gamers to integrate their games into the service, as well as download games that support the service natively.
Having the full support of pretty much every major publisher that makes PC titles except Blizzard and Electronic Arts (post-Dragon Age II, when they launched their competing service "Origin"), the service makes it easier to download and play PC games, having over ten thousand titles available for download, many of which are classic games in their own right. It also boasts more than 30 million unique users.
The main draw, however, is Steam's popular seasonal sales, days-long events that see prices on scads of games slashed by 50%, 75%, and even 90%. Even bundles - a discount in their own right - can go on sale. With even recent indie and AAA titles discounted, many users have given up on paying full price for any game while others make purchases on impulse that they normally wouldn't look twice at. This is causing most of Steam's competitors to follow suit, further encouraging the 'wait for a sale' mentality. It's reached a point where 'significantly cheaper games' is becoming a major argument in favor of PC gaming.
In 2011, Steam added a Workshop feature that allows players to submit any Game Mod that they created to the Workshop so that other people can subscribe to that content and have said content automatically be installed to their game the next time the game is launched. The Workshop also allows people that subscribe to mods have them updated whenever the author updates the files, which is then updated when the game is launched. The Workshop makes downloading and distributing game mods easier for everyone.
In 2012, Steam branched out into distribution of non-gaming software, and also introduced a system called Steam Greenlight to allow the gamer community to submit indie games and vote on whether Steam should distribute them. After trolls tried to submit pornography and such things as a fly-your-plane-into-the-World-Trade-Center game, coupled with people making requests of intellectual properties of different companies, Valve instituted a $100 submission charge, sending the proceeds to the Child's Play charity. It is important to note that this was per developer, not per game, and within a year sleazy developers were bribing players for approval, making shoddy games running entirely on Unity assets, and packing pages of the new releases with what was essentially the same game. Good games drowned in Sturgeon's Law.
In 2013, Steam introduced RPG Elements and a Collectible Card Game, of all things. Shortly before its 2013 Summer Sale, trading cards were introduced to several games (with the Summer Sale, not coincidentally, introducing them to many more), which are collected by playing (or sitting at the main menu of) the game in question. A full set of these cards (which require either trading with friends or a trip to the Steam Market to pick them up from other Steam users to obtain) earns you a badge based on that game and Steam XP, which increases your Steam level. Increasing your Steam Level is largely a Bragging Rights Reward, but also unlocks friends list slots and profile customization options. In addition, themed profile backgrounds and emoticons can be earned by crafting game badges. The community is split into a few camps on this feature: Some decry it as a gimmicky cash-grab, some have taken to playing the market for cash, some have given a collective shrug, and others have eagerly dove into the new feature, now finally having motivation to play the extensive games collections they purchased during Steam's deep-discount periods.
On September 23, 2013, Steam announced SteamOS, a Linux distro focused on gaming on the TV, then further announced the production of "Steam Machines", dedicated gaming hardware which comes with SteamOS by default (though other operating systems can be installed if the user desires). In addition, Valve also announced a new touch pad-based controller which is capable of being used on SteamOS (both on and off of Steam Machines) as well as the existing Steam clients on Windows, Mac, and Linux, which includes an intended total compatibility with every game available on Steam. The Steam Controller and Steam Machines (including the Steam Link, which is a device that allows streaming from a gaming PC to a television) released in November 2015.
On December 2, 2014, Steam added a built-in broadcasting feature, allowing users to spectate other Steam users' game sessions. In 2015, Steam branched into film distribution as well, with them getting a deal with Lionsgate in April 2016. In March 2017, they revealed a deal with Crunchyroll to distribute select anime as well.
In February 2017, Steam Greenlight was scrapped in favour of Steam Direct. While Greenlight's quality control had long been...non existant to say the least, it has been noted that Direct approves even more games than Greenlight, and effectively removes any attempt at quality control, however half-hearted, since Direct is almost entirely automated. Games that are ridiculously broken or deliberately offensive (such as simulating a school shooting - badly) are becoming increasingly common. Additionally, the opinion that Valve is not as greedy as its competitors is rapidly becoming viewed as 'naive'.
On March 2018, Valve announced a successor to the Steam Link, by taking the idea to its logical endpoint, and making it an Android app, with an iOS app being delayed due to Apple not allowing for game purchasing in apps that they themselves don't sell. The app is currently in Beta. On March 2019, a year after the android apps release, "Steam Link Anywhere" was introduced into beta, as a way of playing games from your Steam PC from anywhere that has an internet connection, be it an internet cafe, local library, or a restaurant.
On June 6th, 2018, Valve announced they will now allow anything that isn't illegal or trolling to be on Steam, with mixed reactions from both its fans and critics, with some seeing this as letting down their customers by giving up on quality and others seeing it as standing by their customers by supporting free speech.
On August 21st, 2018, Valve released an upgrade to its "Steam Play" feature for the Linux portion of its audience, in that it can theoretically let you play any Windows game ever released to the platform! Steam Play bundles in a fork of the Wine project, a Direct3D 10/ 11/ 12 compatibility layer called DXVK, as well as a few custom fixes from both projects to make this "magic sauce" work in harmony. Valve has called this magic package "Proton", and released it alongside 21 officially supported games, such as ''DOOM (2016)'' and NieR: Automata. They also made Proton an open-source project for good measure, so others can contribute ideas and code if need be and integrated to run non-Steam games in Linux.
On October 2018, continuing the controversy resulting from the "allow anything that isn't illegal or trolling to be on Steam" including embarrassing incidents in around July and August where Valve pulls bitcoin miners disguised as cheap games snuck into the platform after several Youtubers discovered it, Valve purged hundreds of what they called "game-shaped objects" or what most people know as Shovelware.
Of course, we have a Steam Group right here. Feel free to join.