Left 4 Dead 2/Modes

Co-op campaign
Up to four human players can control the four survivors, with the goal of making it to the safe house at the end of every level.

Single player
Single player is just like the co-op campaign, except the other three survivors are always computer controlled.

Versus
Versus is like the co-op campaign, except an additional group of up to four can control bodies of the infected. The infected try to stop the survivors from making it to the safe house, and points are accumulated for doing better at the assigned roles.

Survival
In survival mode there is no end to the level; your team just tries to stay alive as long as possible.
 * Bronze medal: 4 minutes
 * Silver medal: 7 minutes
 * Gold medal: 10 minutes

Scavenge
Scavenge is another team mode, where one team plays as survivors and the other as infected. The survivors are trying to find fuel cans to fill up their objective, and the infected are trying to kill the survivors. The round ends when the time runs out or when all the survivors are dead. Ever fuel can collected adds 20 seconds to the countdown.

Realism
Realism is the co-op campaign with a more realistic approach. There is no seeing through walls, objects and people aren't outlined, there are no rescue closets and witches can kill you instantly.