Steam is developed, driven, and runned by Valve which collab with other game companies to release their products out through the Steam Store. Theres alot of things you can do in Steam like Community, Store, Forum, News and Workshop which provides you alot of gaming related things. Check Steam at www.steampowered.com or www.steamcommunity.com to see the separate things! Also it's quite handy dandy to run games which you have that can probably be runned by Steam Cloud or so. Look up Steam at wikipedia.