Is a 4 player co-op game. The four players a trying to wade their way through an endless horde of "Infected" humans. The infected humans are much like the ones from the novell I Am Legend. They are very much alive, intelligent, and can work together. The infected in the book "I Am Legend" play more of a vampiric role. With a few that are even zombies but the living infected kill off the undead infected.
Left 4 Dead is the same model with some tweaks. You aren't all alone as in Neville's case both in the fact that you play with 3 others and there are other non-infected humans elsewhere. Your goal as a survivor is to make it to the evacuation point and call in an evacuation. This could take an hour or more to reach this point. While an endless horde of infected standing between you and your goal. The infected are alive and intelligent. While they may not be smart enough to wield guns (yet) they will alert other infected to your presence and begin the assault on your location. Thankfully they are human after all and are pretty weak with a few exceptions. Some infected sport inhuman mutations that make them deadlier but they still die easy enough if you can survive long enough. In certain game modes you and others are even allowed to play as the "mutated"(boss) infected and attack the survivors yourself.