It's about the planet Reach. All of it will be a prequal to the trilogy, so it takes place before Halo 1. It is going to be it's own compact disk (well it should be if Bungie has any knowledge of what their fans want).
Reach is invaded by Covenant, it's where most of the Spartans are, we will be playing as Spartans picture^^, and Reach is glassed by the Covenant, and it is based of the book Fall of Reach.
That's basicly what we know.