Steam is sort of like an app-store for games which only ran on Windows for a long time. You buy games on Steam and can then download the games on any computer you install Steam on. There's a bunch of other stuff such as being able to talk to your friends, multiplayer, and keeping track of achievements. Valve (the creators of Steam) are porting it to Mac and Linux. This is not, however, sufficient to play all games on Steam, since games from many companies are on Steam and not all of them are interested in making their games cross-platform. So, in addition, Valve is porting their Source engine, which powers games such as Counter Strike, Team Fortress 2, Left 4 Dead, and Portal to Mac and Linux so that those players will able to play them. They will of course be able to play any other games on Steam that are class-platform.
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Steam is sort of like an app-store for games which only ran on Windows for a long time. You buy games on Steam and can then download the games on any computer you install Steam on. There's a bunch of other stuff such as being able to talk to your friends, multiplayer, and keeping track of achievements. Valve (the creators of Steam) are porting it to Mac and Linux. This is not, however, sufficient to play all games on Steam, since games from many companies are on Steam and not all of them are interested in making their games cross-platform. So, in addition, Valve is porting their Source engine, which powers games such as Counter Strike, Team Fortress 2, Left 4 Dead, and Portal to Mac and Linux so that those players will able to play them. They will of course be able to play any other games on Steam that are class-platform.