From Valve website: "Steam is a broadband business platform for direct software delivery and content management. At its core, Steam is a distributed file system and shared set of technology components that can be implemented into any software application. With Steam, developers are given integrated tools for direct-content publishing, flexible billing, ensured-version control, anti-cheating, anti-piracy, and more. Steam consumers enjoy the benefit of starting their favorite applications within minutes of confirming their purchase. They can access their applications from any PC. They are no longer challenged to find the latest updates for these applications. And they no longer need to wonder if their device drivers are compatible with the latest software. The Steam SDK also includes an integrated set of communications tools and Valve's Graphic User Interface (V-GUI) that provide built-in support for a variety of services such as instant messaging, configuration, and server browsing."
Steam is a content platform for Half-Life 2, Half-Life 2 Deathmatch, Counter-Strike: Source, Counter-Strike 1.6, and all mods made with either the Half-Life or Source engines. All games made to work with these engines are compatible with Wine.