Gameplay Modes

There are three confirmed Gameplay Modes in Left 4 Dead.

Campaign
You and three other players work together to try to reach the 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 the survivors make it to the safehouse 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 uninfected, 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 human, make it to the end of the level. And if you are a zombie, to prevent the humans from doing just that. Four human players play against four boss infected, which are all 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.

You choose a specific campaign to play, then the first team plays as The Survivors while as the others play as the boss infected. The map lasts until The Survivors reach the end or they are killed. Then the teams switch then spots and replay the level as the opposite team. After both teams have finished the map, the process begins all over in the next level. At the end of each round teams are compared based on if they made it to the end, how much damage they did, what was their health at the end, how many made it to the safe room etc.