Left 4 Dead is an online cooperative First Person Shooter developed by Turtle Rock Studios (a subsidiary of Valve software) and published by the latter for the PC and Xbox 360.
Left 4 Dead pits the player as a survivor agaisnt an ongoing, large-scale zombie apocalypse that has destroyed most of civilization. Luckily, the survivors aren't alone, as you will be accompained by three other survivors, who must cooperate with you (and vice-versa) to complete movie-like experiences, of similar length, called campaigns.
The game has a dynamic, intelligent "entity" present in the game, called the Director, which spawns zombies, boss zombies and helpful items along the journey to help or harm the survivors (generally harm) depending on their performance. This system is entirely dynamic and is designed to work with every map with a special mesh called a nodegraph. There are videos on Youtube which display maps from Counter-Strike: Source being played in the Left 4 Dead demo, entirely functional and playable. The Director controls practically everything in the campaigns, so no playthroughs are alike. This makes the game a different experience every time it is played.
The four survivors have a decent arsenal with which punish zombies with. From assault rifles to home-made pipebombs that attract the zombies and then explode, there is something for everyone. However, the Director also has a few tricks up its sleeves. There are five distinctive "boss" zombies, or Special Infected, that are more powerful than the normal, silly zombies, and have unique abilities. For instance, one known as the Boomer can vomit bile on the survivors, attracting a large horde of zombies. Another one called the Smoker has an extremely long tongue, which he can use to snare survivors and drag them towards him.
Left 4 Dead shipped with four campaigns, varied in setting and overall gameplay. There is a mode known as Versus, which is great if you want to experience being on "the other side". In Versus mode, there're 2 teams of four people each. The teams play every map in a campaign twice: once as a survivor, once as one of the respawning Special Infected. As a Special Infected, tactics and teamwork become immensely important. Unlike the normal, cooperative gamemode, Versus mode's objective is to earn more points than the rival team, by doing well as a survivor and by preventing the other team from doing well as survivors.
This game is very mature in content, with plenty of blood and gore. If you have a strong stomach (or just don't care about those things), then Left 4 Dead is an spectacular and incredibly fun game, perfect to play with a group of friends and some Fanta. Highly recommended.