Source: http://store.steampowered.com/livingroom/SteamOS/

Steam is coming to a new operating system
As we’ve been working on bringing Steam to the living room, we’ve come to the conclusion that the
environment best suited to delivering value to customers is an operating system built around Steam itself.
SteamOS combines the rock-solid architecture of Linux with a gaming experience built for the big screen.
It will be available soon as a free stand-alone operating system for living room machines.

Living room & Steam
Finally, you don’t have to give up your favorite games, your online friends, and all the Steam features you love just to play on the big screen. SteamOS, running on any living room machine, will provide access to the best games and user-generated content available.

Fast forward
In SteamOS, we have achieved significant performance increases in graphics processing, and we’re now targeting audio performance and reductions in input latency at the operating system level. Game developers are already taking advantage of these gains as they target SteamOS for their new releases.

Cooperating system
Steam is not a one-way content broadcast channel, it’s a collaborative many-to-many entertainment platform, in which each participant is a multiplier of the experience for everyone else. With SteamOS, “openness” means that the hardware industry can iterate in the living room at a much faster pace than they’ve been able to. Content creators can connect directly to their customers. Users can alter or replace any part of the software or hardware they want. Gamers are empowered to join in the creation of the games they love. SteamOS will continue to evolve, but will remain an environment designed to foster these kinds of innovation.