Gameplay Modes

There are 5 Gameplay Modes in Left 4 Dead, and 7 Gameplay Modes in Left 4 Dead 2.

Campaign
During Campaign, also known as 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. There is no backtracking to previous levels, so once the checkpoint is reached, you cannot go back.

Single-player
Single-player is a gameplay mode with only one player controlled character. The player assumes control of one of the Survivors, and all the other characters are AI controlled. No other player can join a Single-player game. Other than that, the same rules as campaign apply here.

Split Screen
In 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 Versus, players work together to complete your objectives as both the Survivors and the Infected. 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 Special 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 Special 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 Special 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 or Survivors, review some tactics.

A hidden statistics screen was discovered using the console on the PC version of Left 4 Dead. It shows how much damage you did to specific Survivors, and information specific to the Infected you were before you died. It is not implemented Versus mode.

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 on screen. 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 Left 4 Dead'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:

$$( P + H ) × N × S $$

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

H = Health Bonus. The Health bonus is calculate with the following rule:
 * 1) Each Survivor scores at most 50 points of health bonus.
 * 2) If the Survivor has pain pills in his/her inventory, they will be automatically applied (prior to health kit) to the owner as temporary health.
 * 3) If the Survivor has a health kit with him/her, it will also be automatically applied to the owner. This also overrides temporary health the Survivor has.
 * 4) The health bonus for each Survivor is calculated with following formula:
 * 5) *$$MyHealthBonus = floor( Permanent Health / 2 ) + floor( Temporary Health / 4 )$$
 * 6) Add all four Survivor's health bonus, this became the health bonus recorded

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 don't make it to the safe room, 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
Main article: Survival Mode

Survival Mode was released in the "Survival Pack" which came out on April 21st, 2009. It features sixteen maps, fifteen of which are 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.

Scavenge Mode
Main article: Scavenge Mode

Scavenge Mode is a new gameplay mode announced for Left 4 Dead 2. The Survivors have 2 minutes to find 16 gas cans hidden around a map. For each gas can they retrieve they gain an extra 20 seconds. The goal is to fill a generator with the gas found amongst the 16 canisters while fighting against player-controlled Special Infected much like in a versus game.

Realism Mode
Realism mode removes the auras usually shown around players through walls and around items, making the game require more communication and teamwork. Realism mode also makes bodyshots less effective at dispatching Infected, while making headshots more effective, making headshotting the only efficient way to kill infected. It can be played on any difficulty. Rescue closets are also removed, making the defibrillator the only way to revive teammates. Furthermore, there are a lot more witches wandering around, making it far more risky to fire your weapon haphazardly. Between the dependence on headshots and the number of witches around, melee weapons are even more useful than usual.

Behind the Scenes
Originally, the only mode was to be Versus. However, Valve found that it was very difficult to balance the game so that both teams could have fun, so they split the game modes.

In Versus, the player had the ability to select which Infected he wanted to play as. This menu can be seen in early footage.