Left 4 Dead is a co-operative zombie survival shooter. The ultimate goal of the four humans is to make it through the saferooms in the various levels, and ultimately escape to the military evac zone. In versus mode, there are several human controlled "boss infected" with abilities ranging from the smoker's 100-foot long tongue, to the boomer's horde-attracting bile, but the rager-style zombies are all AI controlled. The "AI director" adjusts enemy spawn, ambush points, and item locations in relation to how well the survivors are faring: if they're doing well, enemies will pour on and on, where as if the humans are barely fighting, the enemy spawn rate would go down and more items could be found. This, both on paper and in practice, that no two games will be the same.
Friendly fire is always on, and it the game keeps track of all positive and negative achievements of the individual player, viewable at the end of each game.

The demo was released on November 6th, 2008 and the full game on the 18th. The game has received positive reviews, with many saying that it is the multiplayer game of the year.



Survivor Gameplay
The survivors are the four human players in L4D. In the beginning of each campaign, the survivors are able to choose between a pump shotgun or uzi, and each get a single med kit. Throughout the levels,safehouses (which function as checkpoints and re-spawn areas) can be found, stocked with ammo and healthkits. In the last stage of a level, the survivors are allowed to set up barricades, explosive barrels and stationary weapons. When the escape is activated (signaling a helicopter, trying to start a car and such), the players must fend off waves of infected for a period of time. Human players are "knocked down" when out of HP and can only use pistols. Unless they are helped up by another survivor, they will bleed out (sped up by enemy attacks) and die for the duration (they can also die if knocked down 3 times). Dead players must be found in re-spawn zones, such as a closet. These survivors will yet out, and have an orange outline to assist their teammates in finding them. Survivors can see the outlines of their fellow humans through the walls, and will automatically yell to alert their teammates, such as "reloading "help me up" or "I see a hunter" as well as manual voice commands.

There are four survivors-

1. Bill - A Vietnam veteran with combat skills that have helped keep him alive, he is also the eldest of the band of survivors. It is stated that he served two tours in Vietnam, and the emblem on his beret resembles that of the1st special forces group.
   
2. Zoey - A college student. Although her professors frequently lectured her for watching horror films instead of studying, it is implied that her knowledge of zombie horror film cliches has kept her alive while her professors perished.
3. Louis - A junior systems analyst at a large company. In-game dialogue suggests he frequented rifle ranges before the outbreak, something his co-workers would tease him about. His clothes and job are purposely similar to those of Shaun from Shaun of the Dead.
   
4. Francis - A tattoo covered biker, who seems knows Bill well.

(This was a concept picture, the in game characters have slight changes, but for the most part are similar.
Zoey is noticeably different, her finalized in-game model having a ponytail and lacking the headband.)

Available items:
Certain items will be available to you at the starting safe house, additional medical supplies, explosives, and alternative weapons can be found throughout the level. You can hold one of each type of item, except pistols (although they'll still be counted as a single item)

Tier 1 weapons (Starting Weapons)
Pistol (single stack 1911, likely Springfield Armory)
Each survivor starts with a single light-equipped 1911 pistol. They hold 15 rounds and are fairly accurate. A second one can be picked up (missing the tac light and rail) to increase the rate of fire and doubling the ammo supply. The pistols can take out ordinary infected in two hits. Even though they are meant as emergency weapons, they make a decent alternative for shotgun and hunting rifle users. They are the only hand held weapons to have infinite ammo, although reloading is still necessary.
The in game model resembles a somewhat odd hybrid of a modern frame with an original 1911A1 style slide.
In game, the trademarks state they are made by the fictional "Finleyville Armory", likely a reference to Springfield Armory.


Uzi
The uzi is the automatic starting weapon and holds 50 rounds in the magazine with 480 maximum in reserve. It fires rapidly in full automatic and is fairly accurate, even at range. Newer players may be better off using the uzi, and it is typically less likely to hit teammates then the shotgun.

Pump Shotgun (Remington 870)
The first tier choice of most players, the shotgun can hold its ground against hordes of infected, and most of the boss infected. It holds 8 rounds in a underbarrel magazine tube with 128 reserve. Both shotguns will have an additional chambering animation if one reloads on an empty chamber. It sprays 10 pellets in a cone shaped blast, and is somewhat more effective at range then most games. However, it still works the best in close range engagements, and it is recommended that shotgun users use their pistol(s) at range.
The trademarks on the in-game texture say it is the "Renegade Rangemaster", probably meant to sound similarly to the very common Remington Wingmaster.

Tier 2 Weaponry (found weapons)
Assault Rifle (M16A3)
Essentially an upgraded version of the uzi, the M16 has the same capacity paired with improved accuracy and power. It is the best all-round weapon in L4D, retaining impressive firepower at all ranges. However, it has 360 spare rounds compared to the uzi's 480.
The ingame texture says it is an M16A2 by the fictional American Armory company, but its full auto feature indicates that it is a M16A3.

Hunting Rifle (Ruger Mini 14)
The primary ranged weapon in L4D, the hunting rifle is the only scoped weapon in-game. It has a 15 round magazine and 180 rounds of spare ammo.
The shots have been tested ingame to penetrate an infinite number of enemies, and will pierce most of the scenery. It will kill infected in a single hit and often blow off limbs. However, it is less effective against the tank then either the M16 or autoshotgun due to its low rate of fire


Auto Shotgun(Benelli M4 Super 90)
Basically an upgraded shotgun, the Benelli fires 12 pellets per shell compared to the pump shotgun's 10, and carries 10 rounds. It is semi-automatic, and fires one shell with every trigger pull. In all other respects, it is identical to the pump shotgun and has an identical ammo pool of 128 rounds. In close quarters, the autoshotgun reigns supreme, and it is one of the easiest weapons to kill a tank or witch with.
In game, they are supposedly manufactured by Rapidarms, a fictional German firearms manufacturer. The stock is collapsible, so it is either a military M1014, or a civilian M11707 shotgun (this is visible when you switch to the autoshotgun).

Explosives (You can only hold one explosive at a time.)
-Unlike most games, the grenades in L4D lack an arming animation, and are simply tossed when the fire button is pressed. They fuses are also lit when they are drawn, and burn indefinitely.

Pipe Bomb
These powerful home-made explosives function as the hand grenades in L4D, with an additional diversionary capacity. They are essentially PVC pipes filled with explosive compounds and capped with an open fuse. Various noisemakers and flashing lights attract normal infected to the pipe bomb, and beeping intensifies in speed as it gets closer to detonation. Bosses, however, will ignore pipe bombs and the beeping will give them away to human players.

Molotov Cocktail
The molotov is a simple incendiary weapon made by filling a glass bottle with gas, and fusing it with a flaming rag. They burst on contact and cover an area in flames for a short period of time. They are ideal for taking out of an oncoming horde of infected or taking down a tank.

Gas Can, Propane Tank, Oxygen Tank
These are carry items instead of weapons, and can be picked up regardless of the player's inventory. They can be tossed by pressing the fire button, or dropped in place by pressing "use" (X for X360 and E for PC). Finales and crescendos are littered with these items, and they can be instrumental in stopping infected. Propane cans explode instantly when shot, where oxygen cans have a short delay after being shot. They hiss audibly as oxygen escapes, and can be "canceled" by picking them up again. However, you can be shot while holding them, which has a chance of setting starting the delay and detonating. Gas cans are similar, but burst into flames when shot. The splash area of the gas is slightly larger then that of the molotov cocktail.


Medical Items
First Aid Kit
These one use kits can be used to heal up to 80 health points, depending on the amount of damage the player has.
First Aid Kit's are rather rare, so they shouldn't be used lightly. Sometimes referred to as "hard health" as it doesn't tick down.
On expert, they only heal 40 points and are only found in safe rooms.

Pain Pills (acetaminophen)
Gives the user temporary health, maxing out their health bar and boosting speed and strength.
Temporary health goes down over time, but can save a player who is in a jam. Sometimes referred to as "soft health" since it ticks down.

Melee attack
All guns have a melee attack. Although it is fairly weak, it is still very useful. Its ability to push thing back means that you can knock away a crowd (infected can fall into other infected and create a domino effect) or knock a teammate out of the way of an attack. It is also one of the few ways to knock a boomer back without killing it.

Infected
The infected in left 4 dead are running type zombies, and come in massive waves. Although these zombies are, according to Max Brooks' accounts of the undead, inaccurate, one must admit that their rapid speed makes them a much greater challenge to survive against than their slow, real-life counterparts. They have night vision and can see the locations or the survivors. Most infected can also sneak through air vents and other passages to ambush the survivors. You cannot play as a standard infected, but there are several "boss infected" you can play as:

1. The Hunter- Fast, agile, and can leap great distances. The Hunter can leap on a survivor, knocking them down and beating them to incapacitation unless killed or melee'd by another survivor. The Hunter cannot take very much punishment however, a short controlled burst can put a Hunter down for good. Two are available at a time, where all other boss infected can only have one at a time.
2. The Smoker - The Smoker is one of the strangest, yet still deadly bosses. They can grab Survivors with their long tongues (which seem to have an incredible distance, upwards of ten yards or more). Unless immediately dispatched, the Survivor will be pinned and unable to fight back. Meleeing the grabbed player knocks them lose. Smokers cannot take very much punishment; perhaps even less than a Hunter.
   
3. The Boomer - The Boomer is rather slow, yet dangerous boss. While they're easily dispatched with a single shot at a distance; in close-quarters the Boomer is a deadly target. They "puke" on survivors, distorting their vision and summoning several dozen normal infected that attack whoever the Boomer hit. When they die, the Boomer explodes, hitting every survivor in range and summoning a horde of infected to attack. (I'm not sure if each survivor hit summons their own horde of infected when hit.)
   
4. The Tank - Deceptively Fast, Strong, and HUGE. They can crash through walls, batter down doors, and send survivors flying. You cannot choose to play as a tank, but you will be prompted randomly: all players will get a chance before any player uses the tank a 2nd time. In the final stage of a level, there are two tanks. There is a glaring disadvantage, however: if the tank does not see a survivor on screen, a "rage meter" will fill up, and when it tops off, the tank will go berserk and the player will lose control. The tank has massive amounts of health and takes several magazines of ammunition from tier two weapons to kill. The tank can throw survivors great distances, throw large concrete chunks, and smash through certain obstacles. When struck by a molotov or lit on fire, the tank does not take damage, but starts a 40 second timer. Regardless of health, this timer will instantly kill the tank when it expires. This matter is much debated in the game community, as it enables the survivors to simply light the tank on fire and run.
   
5. The Witch - The witch is the second hardest boss the survivors will encounter. She is only surpassed by the tank due to her lower health. A single hit from a Witch instantly incapacitates a survivor and instantly kills a downed survivor. Even after knocking down a survivor, she almost always stays and kills the downed survivor. She is extremely fast and very difficult to kite. Fortunately, the Witch only spawns once (and sometimes not even once) a map. She is AI-controlled and only attacks if a player shoots her, aims their flashlight at her, or steps to close. Sometimes the Witch cannot be avoided. The autoshotgun however, will kill her relatively quickly, and a well placed headshot will instantly kill her with it.
   

All of the special infected have special musical cues when they are near.*

Campaigns (official descriptions)
No Mercy
The No Mercy campaign finds you on the rooftop of an obliterated apartment complex, deep in the heart of the once-thriving metropolis of Fairfield—now completely overrun with hordes of Infected. A voice from a bullhorn calls out from a rescue helicopter: Get to the roof of nearby Mercy Hospital and be airlifted to safety. Even if it means shooting your way through narrow alleys, open sewers and seething waves of zombies to get there.

Death Toll
Shoot, shove and sprint through a nightmare suburbia consumed by the Infected horde. The Death Toll campaign has your team of survivors making their way down a turnpike littered with abandoned cars to the nearby ghost town of Riverside—the site of a failed stand-off between the last of humanity and a limitless swarm of undead. Fight through the ruins of small town America to the waterfront nearby, where a rescue party can take you upriver to safety.

Dead Air
Guide your team of Survivors out of the downtown core of Newburg's business district, through derelict office floors and still-inhabited cubicles, and out onto the runway of Metro International Airport, where the only thing standing between you and the last plane out of the city is a murderous army of the undead.

Blood Harvest
Plunge through pitch-black forests, stumble blindly through fields of swaying corn, and take aim at the silhouettes leaping at you from the darkness all towards finding a hold-out that might already have been abandoned to the horde.