In L4D, teamwork is a MUST. There are ordinary, common zombies, which are everywhere, are easy to kill. However, the special infected are not so. Three out of the five zombies require at least one (or all, in the case of the Tank) survivor to kill it, not including the survivor being attacked by it.
The special infected seen the most is the Hunter, Boomer, and Smoker. The Hunter is a sweatshirt wearing zombie, who pounces his victims. If you get pounced by him, you're dead unless someone can knock him off or kills him. The Smoker is a zombie with a really long tongue. It, uh... wraps it's tongue around you and drags you, and if you're on a building, and no one saves you in time, you'll fall off and die. If you don't fall off on any buildings, it'll just strangle you to death. The most annoying zombie is the Boomer, the fat lard. This zombie barfs bile on you. If you manage to kill it before that, you still have the risk of the explosion it creates throwing gunk on you. In either case, you'll be blinded momentarily, while being attacked by a hundred zombies that for some reason are attracted to that stuff.
The worst, and not often seen special infected, the Tank and Witch, are terrible. The Tank is very hard to kill, with it's huge, bulging muscles and tiny legs. The best way to kill it is to have all four survivors shooting at it at once. If they can do that without dying, it usually dies very quickly, even on Expert level. The Witch is simply a sad, crying girl on the floor... in her under garments... Getting too close to her makes he angry. The best way to deal with a witch is to avoid her altogether if possible, but if not, I suggest that the person with the lowest health should walk up to her and try to head-shot her. If the person fails, they instantly die from the witch. Then she'll run around screaming her head off until she's either killed or just disappears.
The main goal of the survivors is to, well, survive, as they make their way to each of the four safe-houses, which eventually lead to the Rescue. The Rescue involves sitting around for, say, two or three minutes, (the radio you call for help says they'll be there in ten, so...) whilst you shoot fifty zombies every wave, with one or two breaks in between so you may fight a Tank.
Left 4 Dead is a bit different from other games for two reasons. The first reason is the fact that it's not a single campaign, it's divided into four separate mini campaigns. They're divided into five parts, the four Safe-Houses, and the Finale (Rescue). The reason for the Safe-Houses is because they're what you start at if your entire team dies (even if only one person is still alive, they can continue, until they rescue teammates from the closets they respawn in).
The other factor is the A.I. Director. The Director is what tells the game where the zombies will spawn, as they never appear in the same place each time you play. You can go from one Safe-House to another, and not see any Tanks, but if you played that same level again, you might find one or two. This was added so that each time a level was played, it would be different the next time you played that level.
Left 4 Dead also has two other game types. The first one, that came when the game was released, is Versus Mode. In this there are two teams, Survivors, and Infected. The Infected you play as are the Hunter, Boomer, Smoker, and, sometimes, the Tank. You'll either start as the people or zombies, and once at least one Survivor gets to the Safe-House, or the Infected kill them all, you start as the other team the next round. Once two rounds are over on the same part (or map), it moves on to the next one.
The other game type is Surviver Mode (added April 2009, as an update). In this, you're goal is to survive, duh. You start in any map you choose, and after getting ready and alerting the Hoard (name for the massive groups of zombies), wave after wave of zombies come to attack you, without stopping, though the number decreases when a Tank appears (which are easier to kill than in campaign). You don't move along the campaign in this mode, you only stay in one spot.
I'd add some more, but I think that's really enough.
Director: Let's drop 4 tanks on him...
Left 4 Dead
Looks like possibly the greatest multiplayer action that we, as mortals, shall encounter.
It features whats called the "AI Director", which spawns the infected/zombies from different points on the map every time the game is played.
Many claim this game is actually sex in its video game form.
Left 4 Dead Dude 2: should of been watching your back noob.
Zoey: ALMOST THERE?!?!? ALMOST THERE!
Louis: Get these vampires offa me!
Bill: They're zombies moron.
Duke Nukem: HAIL TO THE KING, BABY!
Louis - a junior system's analyst at his company's IT department. PILLS HERE!
Bill - a former Green Beret (which explains the hat) and 'Nam vet. The leader of the group and has infinite knowledge of combat.
Francis - a tattoo covered biker who tends to be the one to save your ass whenever it's on the line for some reason.
Zoey - a college student and horror film enthusiast. She is the only female of the group which leads to weird moments.
Each campaign, the survivors begin with a M1911 Pistol, which has an unlimited amount of ammo; later on in the campaign, the player can require another pistol, which equals what? Dual pistols beeotch. The pistol is the only weapon you can use when you are incapacitated. Yeah, that really helps. Also when starting out, there will be a table with ammo, med kits, pipe bombs, molotov cocktails and the choice between a pump-action shotgun or an Uzi. Later on you can get more advanced weapons like an automatic shotgun, a hunting rifle, and an assault rifle.
These campaigns have names, No duh. In each campaign, the survivors have to get to a safe house or room, whatever. The goal is to get to an extraction point, where you will receive rescue albeit from an airplane, a boat, a helicopter, etc. Before being rescued, the A.I. Director, who controls when infected attack or not, will send a shitload of infected and special infected to fuck you up. This usually is bearable, as long as the team dumbass doesn't wander off.
Death Toll - a small town and countryside setting, culminating at a dock house.
No Mercy - an urban setting and hospital setting, culminating at a hospital skyscraper helipad.
Dead Air - an airport setting, culminating at the landing runway of the airport.
Blood Harvest - a farm setting, culminating at a farm house. This is the hardest campaign in the game, hands down.
People tend to mistake the enemies for zombies. They aren't zombies, they're infected, but hey, whatever floats you're boat.
Infected - a regular "zombie" or infected, whatever.
Horde - a huge ass group of infected, which are attracted to loud noises and bright lights. Which means DON'T shoot the fucking cars with alarms and DO throw the pipe bomb.
Boomer - special infected who is fat and vomits all over you, which attracts a horde. Whenever you kill him, his fatass explodes, getting blood all over you.
Smoker - special infected who can be heard miles away because of his horrendous cough. His tongue is like a thousand feet long and tries to choke yo bitch ass with it. When killing this enemy, he just vanishes into smoke.
Hunter - special infected with a liking to blue hoodies. The hunter has a distinct shriek, and can jump high into the air and pounce your ass.
Witch - special infected who is a crazy ass bitch. You can tell whenever you are near her, because she is crying. DO NOT be so stupid to believe that she is human. If she is disturbed, she can incapacitate you with one hit. So don't fuck with her, unless you plan on killing her with one shot. What's more fucked up is that in L4D2, she will be able to walk.
Tank - special infected who appears to have taken way too much fucking steroids. He's about over 10 feet tall and always seems to be able to pull concrete out of the ground and throw it at you, even though you could be fighting him on grass, but he'll magically be able to pull concrete out of the ground. He can also punch your ass halfway across the map. You can't avoid him, so get an automatic weapon out and unload on his ass.
Players: Son of a bitch, who is playing as Zoey? YOU DUMBASS
Left 4 Dead
Player 2: OH SHIT TANK!
Player 1: LEAVE ME! RUN!
Left 4 dead