Well, a lot like Halo 3: ODST's Firefight mode, but with new weapons, maps, and armor abilities. Here's seven minutes of gameplay footage from E3:
BOOM video 5633
Halo: Reach is Bungie's final Halo game with the Microsoft-owned 343 Industries picking up the reins for the next game. Reach is due out on September 14 for the Xbox 360.