Killing Floor is an stellar co-op experience for users with less than optimal PC's and high end gaming rigs alike.
The basis is simple, you and up to 5 fellow survivors must take on waves of various kinds of "specimens" (much like zombies) and earn as much cash as possible to keep you and your mates well protected and armed. You earn cash directly by either killing specimens or healing your teammates. At the end of every wave, you have a short time period to run to the trader (location randomly rotated each round) and stock up on armor repairs, grenades, guns, ammo, swords, explosives, and whatever else you need to keep your squad alive. The waves get progressively longer and more difficult until you beat the final wave, upon which you must endure another round where you fight the game's boss called the Patriarch.
In the game, you can choose one of six classes to play as. While the classes don't have anything exclusive that you can use during gameplay, each class provides its own unique bonuses and discounts at the trader. These include the Medic (improved healing, discounts and bonuses on Armor and Medic guns), Support Specialist (improved welding doors, D&B on Shotguns), Sharpshooter (D&B on Pistols and Rifles), Commando (D&B on Grenades and Assault Rifles), Berserker (D&B on Melee Weapons), Firebug (D&B on SMG's and Flamethrowers), and Demolition (D&B on Pipe Bombs, Grenade Launchers, and Light Anti-Tank Weapons or L.A.W's).
Each class have 7 levels to be obtained, with each level increasing the bonuses gained by using that class. These levels are not earned by an XP function, but by earned by accomplishing tasks that are related to the class's purpose (such as getting a certain number of team healing points for Medic).
The game has 22 officially released maps on the main client and is compatible with dozens of frequently used community maps, any of which can be played by joining a server and waiting a short period for the game to download and load the map. In fact, the whitelist of maps is a mix of official and community maps that stretches to a whopping total of 277 maps! The selection of official maps is highly varied from large, open outdoor mountain passes to small the small narrow hallways of a biotics lab. No matter what your taste, I guarantee that there at least one map that you will attach yourself to and be excited every time it wins the map vote at the end of your game.
Weapons are also heavily varied, with anywhere between 3-6 unique weapons designed to be used for any given class.
In closing, Killing Floor is an amazing Survival FPS that Tripwire Interactive has put together that should be something that everyone has in their Steam library. It should not be overlooked simply because of its dated graphics. This is a must own for every PC gamer. You will pour dozens of hours into leveling up your classes and don't worry, the servers are still alive and always packed, even after all these years. Don't miss out on this one folks! Especially if it's on sale!