Left 4 Dead 2/Gameplay

Stages
Stages are set up in a familiar fashion: equipment, ammunition stores, a trigger for the horde, and a safe house at the end. Unlike the first game, Left 4 Dead 2 has much larger stages. In some areas, this offers the survivors alternate routes when faced with certain dangers (e.g. witches).

Left 4 Dead 2 introduces a new challenge: unlimited hordes. During some triggers, you will have to run to another location to disable something that is making noise. Until that thing is turned off, the horde will continue to attack.

Environmental hazards
Throughout stages there may be places where you can fall and take damage or die. Falling off of buildings typically results in your character having to be rescued by another survivor.

Safe houses
Safe houses represent the end of a stage (except for the final stage in a campaign). When you enter a safe house there will be supplies for you to use in the next stage. Using any of the supplies now will carry over into the next stage (i.e. using two of the four med packs will result in only two being available in the next stage).

If all four survivors (or however remaining survivors there are) enter the safe house, closing the door will end the current stage.

Enemies
There are regular zombies, and super zombies. In the fifth campaign, there are also armored zombies (regular zombies that cannot be shot from the front).

Hordes
Hordes are large groups of zombies that are attracted to survivors. They are excited, and run as fast as they can to attack. Mini-hordes, or small groups, appear at random times or when the survivors reach certain points. A Boomer (Special Infected) can summon a horde of common infected by using its special attack & vomiting on survivors.

Crescendo Events
A large horde is triggered by special "Crescendo Events" during a stage. When triggered, mini-hordes will randomly appear in the area (there are usually only a few entry points and these can be learned to better strategize) until the event is over. Some events are timed, and others must be disabled by deactivating an object.

Special Infected
Special Infected represent the dangerous evolutions resulting from the disease. During the regular campaign (solo or co-op), special infected will try to attack the survivors at random times. On higher difficulties the special infected will coordinate their attacks.

There are an unlimited number of special infected in each stage. The longer you take to finish a stage, the more you will have to fight.

In L4D2, there are eight types of special infected:
 * Smoker
 * Boomer
 * Hunter
 * Spitter
 * Jockey
 * Charger
 * Witch
 * Tank