It's one of those never ending fight modes. You pick a level, and just survive as long as you can. The waves get tougher each time, and you get 30 seconds between each wave, so you can buy perks, guns, upgrade the guns, upgrade armour, get turrets, air strikes, or find a new spot to hunker down. I'm only playing by myself, but it's pretty fun. Can't wait to system link it. Two team mates. It's guna be a riot.
So far, on the easy maps, I've had regular guys, attack dogs, suicide bombers, tougher guys, choppers, heavies who just wade through anything, and guys with riot shields. One time I legged it over a bridge and dropped a C4 charge as I did. A dozen guys following me, cross the bridge. Click. Boom. And half were suicide bombers, so proceed with the random exploding ragdolls!