Online gaming store/client/DRM service created by Valve corporation. Games can be bought on Steam, and there are often
game sales and bonus pack deals for promotions (Bioshock sold for $10.00 on Boxing
Day). There is also a social networking aspect a la Facebook, where you can make friends, join gaming groups, etc.
There are a growing number of games which are using Steam for DRM purposes. These games are, once installed, tied directly to Steam and not your
computer. The pro to this is that you can play said game on ANY
computer with Steam installed using your account. Games also update themselves automatically. The con is that is that Steam has to be running for the
game to play, althought offline mode exists.
The other definitons are outdated and don'
t represent the improvements Steam has made over the years.