According to wikipedia, Steam is an Internet-based digital distribution platform developed by Valve Corporation offering digital rights management (DRM), multiplayer, and social networking. Steam provides the user with installation and automatic updating of games on multiple computers, and community features such as friends lists and groups, cloud saving, and in-game voice and chat functionality. The software provides a freely available application programming interface (API) called Steamworks, which developers can use to integrate many of Steam’s functions into their products, including networking and matchmaking, in-game achievements, micro-transactions, and support for user-created content through Steam Workshop.
Though initially developed for use on Microsoft Windows, versions for OS X and Linux operating systems were later released. Chatting and shopping applications for iOS and Android mobile devices have also been written. The Steam website also replicates much of the storefront and social network features of the stand-alone application.