Campaigns

Campaigns (or movies) are story lines consisting of several chapters (levels, maps). At the end of each chapter, players are given awards for how well they did on certain aspects.

Each campaign features a "Finale," in which the Survivors call for help and must face of a gigantic Horde while waiting for rescue to arrive. This includes several waves of Common Infected and Special Infected, as well as two or more Tanks (though, never a Witch). After holding off the infected for a certain period of time, the remaining Survivors (if any) head to the rescue vehicle.

Currently the game has four playable campaigns, with more confirmed to come from Downloadable Content. They are, in chronological order:
 * 1) No Mercy,  focused on moving towards a Hospital for evacuation. Takes place in an apartment building, subway station, sewer, Mercy Hospital, and a Rooftop Finale. The rescue vehicle that saves the Survivors is a News 5 Helicopter.
 * 2) Death Toll takes place in the under workings of an urban city, followed by the city itself, and ends with a Boathouse Finale. The rescue Vehicle that saves the Survivors is a boat, presumably steered by a casual civilian, or at least a non-army official.
 * 3) Dead Air, a campaign focused on getting through an infected greenhouse, through a metropolitan area, an air station terminal, and finally, a last stand at a Runway Finale. Dead Air's rescue vehicle is a C-130 Hercules that has run out of fuel, and the Survivors must wait for it to fuel before they are rescued.
 * 4) Blood Harvest, a campaign that revolves around getting to a military base in a rural area. This campaign begins with a wooded area, continues on to a train yard, and culminates in a Farmhouse Finale reminiscent of the film Night of the Living Dead. A Stryker Armored Personnel Carrier is the rescue vehicle in Blood Harvest's campaign.

Storyline
There is no definitive storyline involving the four campaigns in Left 4 Dead, only a collection of maps where you need to get from Point A to point B. It is unknown if Valve will ever pick up a solid story after the initial cutscene. However, the campaigns can be linked to one another with careful observations, but this does not define it as Left 4 Dead canon—simply speculation, as nothing about it has been confirmed by the developers.