When SteamOS was first announced in September, it was also revealed to be based on Linux. With most games being Windows-only, Valve had to provide a solution for those who want to play those games through a SteamOS device. The solution was In-Home streaming, which allowed a Windows machine to drive the game and stream the video to another device, which would send back control inputs. While that feature is necessary for a Linux-based Steam Machine, it is also being added to the regular Steam client, and Valve has just started sending out the first wave of beta invites.
If you have been invited you will need to update your Steam client to a beta version on the two involved machines. After that you need to sign-in to one account one both machines, which must both be on the same network, and baring any firewall issues, you are ready to start streaming your games.
Through Steam settings you are able to limit bandwidth, framerate, resolution, and even disable hardware encoding. For more information on the feature, you can check out the Steam Support page: In-Home Streaming.