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 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, Left 4 Dead has four campaigns playable in Campaign and Versus, with the fifth only available in Survival and 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) Crash Course takes place directly after the No Mercy Rescue. The News 5 Helicopter crashes in an area close to The Turnpike in Death Toll.   The Helicopter is caused to crash after Zoey kills the pilot who is, in fact, infected. This campaign is focused on delivering a full multiplayer campaign in just 1/2 hour. It is due to be available for download in September 2009.
 * 3) Death Toll takes place from The Turnpike, The Drains, The Church, The Town and the Boathouse Finale. In which the Survivors contact two civilians; John and Amanda Slater. A boat arrives to rescue the Survivors.
 * 4) 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.
 * 5) 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. An M93 Fox Armored Personnel Carrier is the rescue vehicle in Blood Harvest's campaign.
 * 6) The Last Stand, this campaign has only one level, The Lighthouse. This campaign can only be played after downloading the Survival DLC. It is located in the woods, with a large lake off to the side, unreachable by the Survivors. As it can only be played in Survival Mode, there is no rescue vehicle that arrives to rescue the Survivors and always ends with the Survivors' death.

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.

However, it is worth noting that according to the Developer Commentary, the game was originally supposed to be one large campaign, but was later split up. For example, No Mercy and Death Toll were directly linked; in the former's Rooftop Finale, the helicopter pilot reveals that he tried to do a ground pickup, but was bitten by the Infected in the process. Unfortunately, he soon dies, and the helicopter crash-lands on The Turnpike where the Death Toll campaign begins. Though most of the sound clips have been removed from the game, their audio files are still in the game files. They can be heard here. But now it is known that the helicopter crashes in Crash Course which links No Mercy and Death Toll.