what kind of co-op game is this? is it like L4D where you go through levels or something like Horde?
Unlike L4D, which has your group going from point A to point B in a level, in Killing Floor you have immediate access to the entire level right from the start. Each level has indoor and outdoor areas and it's up to you to decide what the best place to defend is. Enemies come in waves that get bigger with each progressive wave. Each level has several waves, that vary based on difficulty and user preferences.
In between waves, the group has a certain amount of time to purchase new weapons and ammo from the trader. To spice things up a bit, the trader is in a different location after each wave. Once all waves have been completed, your group faces the Patriarch, the baddest zombie around. He employs a minigun, rocket launcher, brutal melee weapons, and a ton of health.