So I can pop a 360 disc into the pc with windows 8 on it and it will play?
Or does it play 360 games as in, download a digital copy?
Neither.
From what I can gather, it's just a rebranding of Games for Windows Live to Xbox Live and rather than have it on a per-game basis, it's integrated into the operating system so any game can make use of the features like achievements, cross-game chat, etc. if the devs use the API.