Left 4 Dead is a co-op FPS game for both PC and X-Box 360. It's overall theme is survival horror as you and your friends can control up to four different characters who must fight their way out of the zombie apocalypse through several different campaigns. You may play on your own as a hero, team up with friends to play as the heroes, or play with others online.
The game is set up in a completely fictional universe in which a rage virus had turned everyone into murderous, zombie-like monsters and the four hero are immune to this virus. Every hero has a unique personality as far as what they say, how they interact with one another, and which guns they will choose in AI mode. Opposite the heroes are the "special infected," which can also be played when in Versus Mode. There are six special infected that are playable, Hunter, Smoker, Boomer, Charger, Jockey, and Spitter, and two that are not, the Witch and Tank. These special infected are much more powerful than the common infected and have unique abilities that cause a lot of trouble for the heroes. There are also uncommon infected, which are slightly stronger in attack and defense than common infected. The uncommon infected can only be found in certain campaigns (i.e. the CEDA worker only appears in Dead Center, Clown Infected in Dark Carnival, Mud Men in Swamp Fever, etc). There are currently seven different uncommon infected.
The overall gameplay is very simple and doesn't change between
campaigns, but each campaign has a unique set up and side-quests along
with a completely random enemy spawn, meaning you won't get bored
repeating a campaign. Each campaign can be accessed through the menu in
any order, but should be played in order at least once to understand
the story in the cut-scenes at the beginning and ending of each. Each campaign has several check points in which the game will auto-save and the heroes may heal up and restock on ammo before continuing to the next section. There is a small "finale" at the end of each in which the heroes must perform a task or defend a location for a certain amount of time, all while battling waves of zombies (called 'hordes'), an increased number of special infected, and several Tanks. At least one hero must survive this to pass the level.
The constant updates to the game and changes within the game itself mean that the game never gets stale; there's always something new to try or discover.