You run through levels, shoot the zombies (fast zombies that run quickly at you... in very, VERY large numbers), occasionally get attacked by some "special" zombies. You have 4 players (or 1-4 players, with 3-0 computer controlled "players). Your mission? Get to the end of the level.
Simple. And damn fun; the enemies are pretty much randomly spawned. Just because you got attacked by 20+ zombies in a small hallway last time, doesn't mean you will this time. And just to ruin your day, if you do good/decent, the game throws a Tank (Special zombie; fast, VERY powerful, and is a giant bullet sponge) at you. End of the level generally is a "survive until rescue gets here".
The game is fun. Not alone, but it's not meant to be played alone, to be honest. A blast with friends.