Horde mode is the star of the Gears of War 2 multiplayer lineup. Oh sure, you've got your classic death matches and capture the flag bullshit (and that co-op mode is still insanely solid); but Horde mode is where it's at.
You and four other players pick a Gears character and choose weapons. Then you're dropped on a relatively small map littered with ammo and perfect areas for cover, sniping, and dying like an idiot when you get cut off from your group.
The idea is to survive wave after wave of Locust hordes that the game throws at you. You lose if every member on your team is killed. You win when you kill every Locust in a wave of the horde – after that, the game pauses to show you which players had the most kills and then chucks you back into the map with another wave.