Left 4 Dead [...] casts up to four "Survivors" in an epic struggle against hordes of swarming zombies and terrifying "Boss Infected" mutants.
Set in the immediate aftermath of the long-awaited zombie apocalypse, L4D's survival co-op game mode is played out across four sprawling "Movies" set in a variety of urban and rural environments. Each "Movie" has an over-riding team objective, is comprised of five large maps, and may be played by 1 to 4 human players.
New technology dubbed "the AI Director" is used to procedurally generate a unique experience each time the game is played. Just as in Hollywood films, the Director calls for new monsters, sounds, and action as it tailors the gameplay experience based upon the team's performance.
[...] Left 4 Dead also includes new multiplayer modes.
* Addictive co-op action gameplay [...]
* Advanced artificial intelligence drives both friendly and unfriendly creatures to dynamically create intense single-player, co-op, and multiplayer experiences every time the game is played
* 20 new maps. 10 new weapons. Four sprawling "Movies"
* Match making, stats, rankings, and awards system drive collaborative play
* Designer's Commentary mode allows gamers to go "behind the scenes" of the game
* Powered by Source and Steam
source: store.steampowered.com/app/500/