Designed for people who want to make gaming-centric computers... making the interface more like that of a console as opposed to a traditional operating system such as Windows and iMac based on linux...

http://www.giantbomb.com/articles/valve-has-its-own-operating-system-now-in-steamos/1100-4746/

So the idea behind it is to have a second box plugged at your tv and stream games from your PC or play the games with linux support natively... or something. I find it interesting seeing a company the size of Valve throwing their own Linux client into the wild, I wonder if it'll catch on? I think it's an indicator of the much rumoured 'Steam Boxes' that are meant to be arriving sometime soon (after all Valve have 2 more anouncements to make this week)