Killing Floor is a coop survival game and Steam are having a free weekend this weekend with the game also knocked down to £3.74, 75% off it's regular price. If you haven't played it before think a generic survival mode with waves of enemies with the option to buy weapons in between each one. I bought this game at least a year ago I would say but never really played it. But now have put a few hours in to it over the last day or so. It is quite fun, especially if you can get a somewhat decent team together. If you do die during a wave of enemies you will remain dead until the next wave where you will respawn with the starting weapons and money, so I would suggest not dying, so long as one member of your team survives you will all respawn, but if everyone wipes it's back to the start again. It's basic survival mode stuff going on here really.
Depending on what weapons you choose you level up in a certain perk making you better with pistols, rifles, shotguns, medicing, melee weapons etc. The more you play the easier it will get if you stay on the same difficulty, I forget the names of them but there is beginner, normal, hard and I believe a couple more after that as well. It is quite a fun little game which I'm pretty sure I've mentioned already and for £4 you can't really go wrong with it. It's got about 48 hours left of the free weekend anyway. If anything give it a shot over the next couple days.