The messaging for this is not great but that's partially because they're trying to do many different things at once.So your saying you could stream windows games from your PC to your steambox?
Why would they make it that way? Doesn't it make more sense to just put windows on the steambox?
What we know so far is that SteamOS is a Linux OS that you can install to a computer. Any typical PC can boot to SteamOS or Windows or dual boot whatever. Valve is claiming SteamOS will be a better OS for gaming for many reasons. Mostly because it's open and faster. The only downside is that the library of games compatible with Linux is incredibly smaller than that of games compatible with Windows. One of the proposed solutions is streaming. It's not ideal but it may work well for gamers that are looking for a second machine. That might work for someone like me for example.
A Steam Machine is basically a sort of Valve "pre certified" PC. They're working with vendors to bring these pre certified machines to consumers. They will probably meet some minimum specs and most likely there will be tiers. For example, Tier 1 might be MSRP $99 and be good for streaming games and playing some 2d or low end 3d games. Tier 2 might be MSRP $200 and play games natively at the 360/PS3/Wii U level. And then Tier 3 might be MSRP $500 and play games natively at the PS4/Xbox One level. That's obviously all conjecture but it's a thought. In any case, the Steam Machine will be able to run Windows if you so choose because it's still just a PC, but it will come pre-loaded with SteamOS. The goal, I think, is that SteamOS will become more and more attractive as people start making Linux builds and ports.