Steam is an online entertainment and distribution platform for gaming, as well as a forum for the gaming community. Once downloaded, the user can use Steam to access 3,500 different games, upload their own content, trade items, or join in the online chat with Steam's 75 million registered users.
Steam, a free-to-use service operated by Valve, launched for Windows in 2003.
Games can be played offline, on your phone or even via your television. Steam also offers automatic patching, so that game updates are processed more smoothly, a voting system for independent games (in the hope of seeing them receive mainstream releases) and the Steam Workshop, a web-based software zone which helps gamers, developers and tinkerers to create, modify and upload their creations for Workshop-compatible games and share those modifications with everyone.
Here's a tutorial to help you get started on Steam: