ZOMBIES!!!!!!
your basic information on your enemies.
The common infected encountered during the game are fast and agile, weak individually, but may be overwhelming in numbers. They display a special attraction to high-pitched alarm-type sounds, such as the beeping device attached to pipe bombs and car alarms. Common infected will often chase down the source of these noises while ignoring lower-pitched, but much louder sounds such as gunfire. They occasionally attack in masses referred to in-game as a "horde".[19]
In addition to the common infected, there are five "special" or "boss" infected whose mutations grant them special attacks that make them much more dangerous:
The Boomer, a bloated infected whose vomit and bile (which may be released at will and upon death) blinds the player briefly and attracts a horde of common infected.
The Hunter, an agile infected that can pounce on survivors from great distances.
The Smoker, an infected that ensnares survivors with its long tongue at a distance and, upon death, releases a cloud of smoke that obscures vision.
The Tank, a gigantic, muscular infected that is powerful, slow and difficult to kill. It can knock players backwards, dealing massive damage, or attack from afar by throwing a lump of debris.
The Witch, a passive female infected, who, when provoked by loud sounds, lights, a survivor approaching too closely, or general stimulation, will trigger her to attack her provoker. She is able to knock down or kill the provoker (depending on the difficulty setting) in one hit. If her victim survives, she will attempt to kill them if not killed herself.[20]
information provided by
http://en.wikipedia.org/wiki/Left_4_Dead