edit: i noticed a lot of you reach this page in search of ways to play left 4 dead via local area network, and i’ve include a guide at the end of this post. hope it helps! feel free to post questions in the form of laughters (comments).
i’m sure most of you first-person-shooter enthusiasts have tried this game, or at least heard of it.

left 4 dead is a co-operative, survival horror, first-person shooter game by valve corporation. the game uses the source game engine (same as the famous counter-strike: source and half-life 2). well, counter-strike was, and still is, an incredibly long-lived global online gaming phenomenon.
anyway, back to topic. left 4 dead pits four survivors of an apocalyptic pandemic against hordes of aggressive infected (zombies). there are 2 game modes — a four-player co-op campaign mode, and an eight-player versus mode. usually, in order to play any pc game with your friends, you have to achieve that through multiplayer/local area network, but this game allows you to do so via single player campaign (which makes it a completely different game).

the survivors in co-op campaign mode
the game focuses alot on cooperation and team play — colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. the ai is generally solid — it’s good about killin zombies, not hittin you with friendly fire, and even patchin you up with med kits if you’re down. however, the downside is that the bots are passive — you always have to be in the lead, which is problematic when you’re hurtin.

team play is very important to survive
the zombie horde that you battle is based on the fast zombies seen in movies like dawn of the dead and 28 weeks later. these zombies don’t shuffle, but instead they sprint at you with inhuman speed, and tear down doors to reach you. horde zombies aren’t bright, but do not look down on them as they are overwhelmin in large numbers.

horde zombies with inhuman speed
in addition to the common infected, there are five special zombies whose mutations grant them special attacks — the hunter, an agile infected that can pounce on survivors from a great distance; the smoker, an infected that ensnares survivors with its long tongue at a distance and, upon death, releases a cloud of smoke; the boomer, a bloated infected whose vomit and bile attract the zombie horde; the tank, a huge muscular infected that is the most powerful and difficult to kill; and the witch, a passive infected that, when provoked by a loud sound or the flashlight, will attack her provoker.

looks like someone startled the witch
in versus mode, four additional players can take control of the special zombies (apart from the witch), which are randomly assigned to them each time they spawn. while in spawn mode, infected players can quickly roam around the map in search of an appropriate place to spawn. this location must be at a certain distance from any survivor, must be out of the line of sight of any survivor, and must be outside restricted areas such as safe rooms. the infected have the ability to see survivors through the walls every time a survivor runs, talks, or fires a weapon.

boomer (what a fugly looking infected)
left 4 dead won several awards from various publications and gaming authorities. ign gave both the pc and xbox 360 version the best online multiplayer game of 2008 award and additionally gave the pc version awards for best use of sound and best shooting game. left 4 dead also won awards from gamespy for the best multiplayer game of the year and spike tv for best multiplayer and pc game. what are you waitin for? grab your copy and start survivin today!
source: wikipedia.org
LAN co-op: first things first, it all revolves around the console. this must first be activated in the “options” menu (go to “keyboard/mouse”, then enable the “allow developer console” option). a restart of the game is required before you can view the console by pressing the tilde (~) key while in game menu or during game.
to host a LAN co-op game: open the console and type “map l4d_[map name]“. for example, to load the very first map, type “map l4d_hospital01_apartment” without the inverted commas (“).
to join a LAN co-op game: open the console and type “join [local host IP]“. for instance, type “join 192.168.1.100″ without the inverted commas (“).