Gameplay Modes

There are five Gameplay Modes in Left 4 Dead.

Campaign
Also called co-op. You and up to three other players work together to try to reach the end of the campaign by fighting your way past large numbers of AI-controlled infected. There are four difficulty levels: Easy, Normal, Advanced and Expert. You unlock a checkpoint if all living survivors make it to the safe room and close the door behind them. You cannot backtrack, so once the checkpoint is reached, you cannot go back.

Single-player
A Gameplay mode with only one player controlled character. The Human assumes control as one of the survivors, and all the other characters are AI controlled. Other than that, the same rules as campaign apply here.

Split Screen
Two players on the same console take control over two Survivors, and the screen is split vertically, although it can be changed to horizontal in the options menu - other than that, this is identical to Single-Player.

This game mode is exclusive to the Xbox 360 version of Left 4 Dead, but may be accessed in the PC version if you have a controller for the second player and modify some files. Here is a guide to how to play splitscreen on the PC. Note that the original poster forgot something to mention in his post. Plugging the Xbox controller in and out during the loading screen is NOT the only way to set the Xbox controller to player 2. During gameplay, you can also unplug and plug the Xbox controller back in within one second to set the controller to player 2. In fact, the second method proves to be more effective with several people.

Versus
In this gameplay mode you work together with other players from Xbox Live or Steam to complete your objectives. That means that if you're a survivor, make it to the end of the level. If you are an Infected you need to prevent the humans from doing just that. Up to four human players can play against a maximum of four boss infected, all of which are controlled by human players. This gameplay mode is currently the only mode where human players can choose to play as the boss infected. Some maps are also slightly changed to make it harder for the survivors, such as removing closets or areas where similar tactics are possible.

The lobby leader chooses a specific campaign to play, then the first team plays as The Survivors while the others play as the boss infected. The map lasts until the Survivors reach the end or they are killed. Then the teams switch roles and replay the level as the opposite team. At the end of each round teams are compared based on criteria including: if the survivors made it to the end, what was the survivors' health at the end, how many survivors made it to the safe room, and how difficult the map itself was. After both teams have finished the map and been scored, the process begins all over in the next level, where the team that has the most total points plays as the survivors first. Once the campaign has been played through entirely this way, the final scores for each team are displayed, and the game returns to the lobby. For help as Infected, see Tactics Playing As Infected

Survivors
Survivors will spawn at the beginning of each chapter as usual, but they will not retain any weapons, health or items from the previous map, and they will begin with tier 1 weapons and 100 health. Survivors do not respawn in closets when they die, unlike the campaign mode.

Infected
The player controlled infected respawn after a certain amount of time which depends on the amount of infected players there are. When there are four infected, players can wait up to 30 seconds to respawn, and a team of one single infected can respawn in around 5 seconds. After waiting out the respawn time, the infected player will enter spawn mode which is indicated by a blue aura onscreen. In this time, the player must choose a location where they want to spawn, and this must be out of sight of survivor players, and a certain distance away from them. The infected player can only run and jump around the map in spawn mode, and they cannot be seen by the Survivor team. Other infected see spawning players as a blue outline. Once the infected player has spawned they are active and play as their infected role to attack the survivors.

Scoring
The scoring formula for L4D's Versus mode has not been made public by Valve, and is often under dispute by players. Arguments often start when players heal themselves or weaker players or swap pills before shutting the safe room door, especially if another player shuts the door before they're done. It's been generally agreed that healing one's self before shutting the door is unnecessary, since teams with low health and 4 medkits will usually score above 160. It's not yet been agreed whether healing a weaker player will help the score and how much if at all pills and temporary health contribute.

A rough formula for a team's score each round is:

$$( P + ( B+ L + Z + F )/2 ) × N × S $$

P = Average distance the survivors traveled, between 0-100%

B, L, Z, & F = Each survivor's total health if they used a medkit, if they have one.

N = Number of players that survived. If the number is zero, the game treats it like a 1 and the score is unchanged.

S = That level's score multiplier, which increases as the game progresses.

Note that if the survivors... aren't, then their score will be P × S. This fact has led many players to "go for distance" when they feel that all hope is lost in a round.

Survival Mode
This gameplay mode was released in the "Survival Pack" which came out on April 21st, 2009. It features sixteen maps which are mostly taken from existing campaign levels, and one new map which has been called The Last Stand. The campaign levels have been edited to remove things such as closets and other small choke points often exploited by the Survivors in Crescendo Events and Finales.