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.