So would I be playing the games through the computer or through the television? Also, would I still be able to use a mouse and keyboard?
I think what it's saying is you can build a home theater PC that you hook up to your TV and put this OS on it. For games that are compatible with this new OS, you would play them off of this SteamOS HTPC.
If you have a game in your library that is Windows only, you can stream that game to your HTPC.
It's based on Linux, so I'm pretty sure you'll be able to use mouse and keyboard if the game you are playing supports it.
Having said all that, I wonder if you'll be able to stream FROM a SteamOS PC to a Windows based installation of Steam.