Left 4 Dead separates the men from the boys with multiplayer that pitches you against hundreds of zombies!
In Left 4 Dead you are a survivor left after an infection that rocked the world. You must fight your way through maps with your friends while battling "boss" zombies and trying to reach a safe house at the end of each level. There are movie campaigns with 5 chapters in each campaign. At the end of each of these campaigns you radio for help and have to hold out in the last safe house until help arrives using anything available to you such as gas tanks or mini guns.
There are 6 types of zombies you will encounter during your adventures. The first is, of course, the common infected. These guys aren't very powerful and only are effective in huge numbers and aren't anything to worry about. The next is the boomer; it's specialty is to vomit all over any surviver who is unfortunate enough to get close, this bile will attract hordes of common infected to maul the survivor. Thirdly, there is a smoker which uses a very long tongue to drag, strangle, and kill (but not necessarily in that order) anyone who gets in its way. Another shady zombie is the hunter. It will lurk in the shadows stalking your group of survivors while you make your way to the safe house. Its only warning is a blood curdling battle cry right before it pounces on an unsuspecting survivor. Arguably the most powerful is the tank. Its power comes from its sheer size and brute force. Tanks will throw massive chunks of cement and try to pile drive you into the ground with car sized fists. Last is the witch with razor sharp claws capable of incapacitating a survivor in a single bloody swing. Take care if you hear the tell tale sobing of the demented witch.
Multiplayer is a blast playing with friends against hundreds of zombies. Survivors are able to save each other when down and trade medical supplies when others need help. Team mates who have been killed can be found in closets later in the game so they can rejoin the team.
Single player is almost identical to multiplayer but is somewhat hampered by incompetent AI that will shoot at witches and get you killed. It appears 3rd on the game's main menu so you can immediately tell the emphasis is on multiplayer in Left 4 Dead.
The bottom line is that this is an amazing game and you should buy it no matter what. The new free DLC adds a new survivor mode that compensates for there only being 4 campaigns. Valve really did something great with this game!