Steam is a digital content distribution platform for games. It wraps up the online store, purchasing, user accounts, multiplayer gaming functionality (with features similar to xbox live), and actually downloading and running games on a user's machine.
It is Valve Software's creation that they use as a channel to distribute their own games (Half-Life, Team Fortress, Left 4 Dead, Portal, etc.) It is one of the best platforms of its kind and a huge number of other game developers have been using it as a release channel for their games as well.
Porting it to Mac OS X and Linux is a big deal because it means those platforms now have a first-class digital content distribution platform for games as well. So, for example, all of those game makers who are already using multi-platform game engines and releasing their games on Mac as well as PC will be able to do so through Steam, rather than through Steam for the PC and some other channel for the Mac.
Additionally, Valve is porting several of their big (multi-million sales) games to the Mac, shoring up the Mac as a legitimate gaming platform. All of this helps spur a self-reinforcing feedback cycle boosting game development on the OS X (more sales, more games, more development of multi-platform game engines, all of which feeds back on itself).
In short, with this one move Valve has pushed OS X (and soon make Linux) from 2nd class citizen to nearly a full peer with Windows as a gaming platform.
It is Valve Software's creation that they use as a channel to distribute their own games (Half-Life, Team Fortress, Left 4 Dead, Portal, etc.) It is one of the best platforms of its kind and a huge number of other game developers have been using it as a release channel for their games as well.
Porting it to Mac OS X and Linux is a big deal because it means those platforms now have a first-class digital content distribution platform for games as well. So, for example, all of those game makers who are already using multi-platform game engines and releasing their games on Mac as well as PC will be able to do so through Steam, rather than through Steam for the PC and some other channel for the Mac.
Additionally, Valve is porting several of their big (multi-million sales) games to the Mac, shoring up the Mac as a legitimate gaming platform. All of this helps spur a self-reinforcing feedback cycle boosting game development on the OS X (more sales, more games, more development of multi-platform game engines, all of which feeds back on itself).
In short, with this one move Valve has pushed OS X (and soon make Linux) from 2nd class citizen to nearly a full peer with Windows as a gaming platform.