Left 4 Dead 2/Modes

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

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.

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. Every 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.

Mutations
Each week, Left 4 Dead will feature a random mutation. These feature a significant change in gameplay, usually to make it more difficult.

Last Man on Earth
Single player mutation. As the only survivor, you need to reach the safe room, but there aren't any common infected to attack you.

Despite most special infected having the ability to pin you down, you will be released and instantly revived as soon as your health is drained. However, you will be critical, and need to use a first aid kit or otherwise be killed by the next special infected.

To deal with enemies, you will need either an accurate weapon or one that guaranteed a quick kill on enemies. This depends on the situation; while a tank may require an auto-shotgun, you may prefer using a sniper rifle to quickly take out hunters or smokers.