Finale

The Finale of a campaign is something akin to a "boss fight" in other games. It is similar to a crescendo event, though on a much grander scale.

Waves
The finale has five waves, and each wave is the same for every campaign. The Witch is the only Special Infected that is never spawned in a wave. Even though the Survivors are given a time-frame for the arrival of the rescue vehicle, it could take longer or shorter than this time depending on how long it takes the Survivors to make it through each wave.

First wave
The first wave does not begin until the Survivors have used the radio to call for help, or have started the fuel pump in the case of the Runway Finale. This wave consists of three to four groups of horde. Smokers, Boomers, and Hunters can appear during this wave. It begins roughly ten seconds after the countdown has been initiated, and is heralded by a loud cry from the Infected.

Second wave
The second wave begins once all of the Common Infected from the first wave are killed. Any Special Infected that are still alive after the first wave will continue to cause the Survivors grief. The second wave consists of an attack from a Tank. No Common Infected will appear during this wave unless an AI or Human controlled Boomer has vomited on you.

Third wave
The third wave is similar to the first, but there are more groups of Horde and the size of each group is larger. Special Infected also appear more frequently during this wave.

Fourth wave
The fourth wave is another attack from a Tank. This Tank usually attacks from a different direction than the first Tank attacked from and the same as the second wave, there is no horde, unless an AI or Human-Controlled Boomer has vomited on you.

Fifth wave
The fifth wave begins after the second Tank has been killed. The rescue vehicle will arrive shortly and the Survivors must fight their way to it. This wave is the largest of the five and consists of several groups of Horde, several Special Infected, and up to two Tanks that all attack at the same time. Since the objective for the Survivors is to make it to the rescue vehicle, killing these Infected will only cause them to respawn to attack again.

If you want to kill more Infected, then do not enter the rescue vehicle. Instead, stay behind and keep fighting. This is a quicker way to get the Zombie Genocidest achievement, which is to kill 53,595 Infected.

Left 4 Dead 2's Finales
During some of Left 4 Dead 2's finales you will be constantly moving. The Infected will be attacking from every direction possible. As you move forward, areas behind you will be made unavailable by various means. You will of course be heavily attacked by the Infected. Tanks seem to have set points along the course to spawn at instead of appearing at a set time, as evidenced by a video of The Parish's finale where the area the Tank was fought had fences around the edges of the bridge to prevent easy death.

Swamp Fever's finale has been described to be like the first game's: defending a certain point from the Infected while waiting for rescue to come to you. Whether any other campaigns will have a finale like this is currently unknown.