It will have new gametypes, Xbox Live matchmaking and deep customisation from the in-game editor. It also supports the new multiplayer bits.
Things like Active Roster, Queue-Joining, 4 player co-op over Live and 2 player split-screen. You can even create and then share your own Firefight modes, which lets you alter game rules like match length, gear loadouts, health, jump height and more.
Halo: Reach releases exclusively on Xbox 360 September 14th.