In L4D(Left Four Dead) you are placed into a survival horror type situation, wherein the entire city is being overrun by infected individuals whom resemble the infected from 28 Days Later(And Weeks Later). They can die, if shot enough, and they're unable to infect your characters due to most likely an immunity that your characters have to the strain.
Since you're in a massive city this means thousands upon thousands of Infected are available to kill you. The most I've ever seen on screen at one time was somewhere in the range of fifty, I believe.
Since you're up against an unending horde of death you're going to have to avoid getting hurt.
Step 1 :: Know your position
Most players who are in the back area will not close doors behind them, will not watch their backside and mostly stay oblivious to everything going on. That's the wrong way to play.
Closing a door slows down hordes and Special Infected. It also gives you a chance to be alerted to their presence, as you can hear them banging on the door. Then, as they are banging on the door they'll form a hole in the door that you can shoot through. This is a major tactical edge.
If there are three teammates in front of you, you'll primarily be hitting them if you try to shoot through them at the zombies ahead. as such, you need to trust and rely on them to handle the basic zombies, unless if you have a clear line of fire(Which rarely occurs in confined areas but can easily happen in larger sections).
Also, if a Horde is coming or a Boomer pukes on you, stick your back to a wall and your sides to a wall if possible. The less area they can attack you from means the greater rate of survival, as you can melee them all away if they're all just in front of you. If you have nowhere to run to, pray to God.
Step 2 :: Know your guns
There are five guns, not counting the mounted minigun, that are available as primary weapons.
The Shotgun and the automatic shotgun are wide-spread that fire in a cone shape. You can shoot zombies that are standing right next to your teammates without hitting the teammates, but you need to aim properly so the cone won't hit your teammate. If you can not do that, then do not use the shotgun. On top of that, the shotgun is actually worst if you're in the back of the pack and the action is happening up front. If you know you're going to be sitting away from the main action, then stick with the other weapons.
The SMG and M16 are automatic weapons which prove highly effective at killing basic zombies. Aim for their head area, spray and you win. However, your ammunition gets eaten up quickly, and so you'll need to learn how to pace your usage of this weapon. Primarily, using this to kill off a single zombie tends to be a waste, conserving the ammo for Special Infected and Hordes is usually the way to go. The Tank can and will laugh at you shooting him with this weapon, as it barely seems to even affect him in comparison to some of the other weapons.
The Hunting Rifle is death incarnate. There have been tales of people playing the Demo and everyone on their team packing this weapon, only to then butcher all that stand in their wake. Able to one-hit kill zombies, and able to be fired rapidly the Hunting Rifle is the end all best weapon for those with extremely high accuracy. If you have poor aim then I recommend against using this.
Pistols and Dual Pistols. "Arlan, is there an advantage to not picking up a second pistol?"
Hell if I know. From what I've seen the accuracy of the Dual and the Single are the same when crouched, and the Dual offers more ammunition before reloading. However, the Single seems to have a faster reload. In the end, though, Dual Pistols make for some of the greatest zombie cleansing weapons out there. The weapon to use when going up against lone zombies.
Step 3 :: Friendly Fire Isn't Friendly
Friendly Fire is permanently on in this game for the Survivors. This means that if you have poor aim then you're going to be saying "Sorry" a lot to your teammates. It also means that if your teammates are reckless and like to run around like it's Counter-Strike then you're going to be groaning a lot. But in the end, what it really means is one simple thing.
Don't shoot if you don't have a clear line of fire.
Don't throw grenades if they might land on your teammates lap(Yes this happens).
BE VERY CAREFUL WITH MOLOTOVS! I can not stress that one enough, I've seen whole teams wiped by a single misplaced fire bomb. They have a massive area of effect so take that into account when tossing it anywhere remotely close to yourself, also make sure to warn your teammates that you're tossing one.
Step 4 :: Running Can Win
I don't get why people do this, but a lot of the time they'll have a clear path to the Safe Room and then they'll stop and try to save one of their teammates who's being eaten alive by a horde. That then gives the Hunter/Boomer/Smoker/whatever a chance at that person.
If you have a clear path to the safe room, you are the last person standing, and there is a horde of zombies behind you RUN FORREST RUN!