http://store.steampowered.com/about/
Digital game distribution for PC, Mac and Linux. It was created by Valve, one of the most respected and successful game creation studios of all-time. Valves is mostly known for Half-Life and the associated mods such as Counter-Strike.
Steam was created to unify users with all of the mods and versions. I remember in college have to go around to friends rooms to make sure they had the latest patches, etc. so we could still all play together. Steam deploys automatic patches, updates, etc. to streamline the process.
As a result, Valve opened up the platform so other studios could release their games via Steam as well.