Gameplay Modes

There are three Gameplay Modes in Left 4 Dead.

Campaign
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.

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.

The player controlled infected spawn 20 seconds after they are killed. They can choose their spawn location and ambush The Survivors easily.

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, how much damage each team did, 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, the process begins all over in the next level. 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.