The city has been infected by a hideous new strain of rabies, and you're one of the few survivors, however, you may not always be helping them. Even though Left 4 Dead is essentially a shooter that's built with the Source Engine (and not part of the upcoming Orange Box) this is not a run and gun shooter. If you don't play alongside the other survivors, and try to Rambo everything - you will die. However, the beauty of Left 4 Dead is that you also have the option of playing on the Infected side, meaning campers and griefers now have a game to call their own.
For gameplay videos head to the media section.