L4D

Boomer

Appearance: An extremely bloated fat man. Huge tumor-like protrusions come out of his face, and his belly button has a large leech-like thing that causes it to ooze blood.

Sounds: A Boomer is one of the loudest Infected in the game, but paradoxically, can be the hardest to recognize. This is because most of his vocalizations are similar to those of regular idle zombies. If they get louder, though, you know you've got a boomer nearby. He makes a vile retching sound when vomiting bile on the players.

Strengths: The Boomer is one of the nastiest infected due to his ability to spawn a Horde. Any player that is vomited on by this abomination will cause the AI director to instantly spawn a Horde of zombies that will chase them (and them alone) down and rip them to shreds. On top of this, any environmental zombies will be attracted as well, adding to the body count. If more players are affected, more zombies are spawned, but with diminishing returns. In other words, four bile-covered players don't spawn four times as many zombies in the Horde. His death explosion also covers players in bile.

Weaknesses: The Boomer is one of the weakest Infected in the game due to his lack of any combat ability whatsoever. His melee attack is ineffectual, he is slow, and falls from tall heights will kill him. He also dies after a single shot, which makes him explode violently due to being so fat and full of gross zombie fluids.

Special: The Boomer will hide around corners, behind pillars, and generally in locations where it is hard to hit him without him barfing on you. If you know where the Boomer is, bash him twice (not three times, or he will explode on you!) so that he stumbles backwards, back up, and blow him away.
Hunter

Appearance: A hoodie-covered Infected that hunches down on all four. The Hunter also slouches when not charging their pounce, making them appear quite a bit like urban punks. This is accentuated by the duct-tape wrapped around random limbs.

Sounds: A Hunter is instantly recognizable due to the screaming sound he makes when leaping. Outside of this, they growl and snarl frequently when meandering about, although it can be hard to hear sometimes. When attacking, the claws make deep rending noises and the victim will scream for help.

Strengths: Hunters are incredibly fast, have a low profile, and can strike from virtually any angle. This makes them one of the hardest Infected to hit unless their weakness is being exploited. They can't be removed from a player that has been pounced on without outside interference. In other words, the player that has been pounced is helpless. They also take a reasonable (but not incredible) amount of damage to kill.

Weaknesses: The delay after a Hunter's leap lets players shoot or stun them, making them much easier to kill. When pounced on a player, they take a moment to begin clawing at them, which also leaves them open to attacks or stuns. Finally, a stun will instantly stop whatever the Hunter is doing (even in mid-leap) and cause them to stumble for a few seconds, leaving them very wide-open.

Special: Hunters are nasty Infected and can ruin any player that is on their own, as they can tear the player apart before the teammates get there. However, if the whole team is in a group, Hunters are a piece of cake. Bashing them off the player they are tearing to shreds is advisable, as stray rounds can (and will) hurt the downed player. Hunters have caused more failed levels than any other Infected, as they will often pounce the last member of a team as they try to save teammates and incapacitate them quickly.
Smoker

Appearance: An average-sized Infected with huge tumors all over its arms and face. Has a very long tongue as well. This tongue can be shed when the Smoker is injured, allowing them to get away.

Sounds: Talk about an anti-smoking agenda. The Smoker makes all sorts of horrible wheezing and coughing sounds when idle. Launching the tongue creates a sort of slurping sound, and the music becomes dramatic whenever a survivor is being strangled to death. When they die, they explode with a puffing noise.

Strengths: The Smoker is the longest-distance Infected and can easily separate groups of survivors through good use of its tongue. When a survivor is being strangled, they can't do anything to free themselves (much like the Hunter's pounce). This forces a teammate to run to the person and free them, potentially causing more problems for the survivors. They have a tendency to find the nooks and crannies where it is hard to see them, climb up on building, or move around in groups of regular infected in order to surprise the survivors.

Weaknesses: In-between the Boomer and Hunter is where the Smoker's durability sits. They can only take a few shots before dying, making it essential for them to catch survivors off guard. When they grab a survivor, the survivor has a few seconds to fire at the Smoker before they become constricted. If they deal enough damage, the Smoker will let go and possibly even die. When constricting a survivor, any other survivor can bash or shoot the tongue or smoker to cause it to release the person they are strangling. Bashing the player counts as bashing the tongue, but shooting them does not.

Special: Smokers are relatively harmless by themselves due to their weak damage output. However, much like the Boomer, they can turn an entire game around. If a smoker grabs and manages to strangle a player, it can force the other teammates to chase the player down and save them. On top of this, they often linger in large groups of common Infected, which will attack the strangled survivor and act as a shield for the Smoker.
Tank

Appearance: Muscle, muscles, and more muscles! The Tank wouldn't look out of place in Gears of War. They are gigantic, drag their knuckles along the ground, and run like apes. If you manage to miss seeing a Tank, you've got to be blind. A hilarious detail: Tanks have no lower jaws, so their tongues just dangle!

Sounds: The Tank snorts, bellows, and generally sounds like an angry rhinoceros. The ground nearby trembles as they move, and the music gets very intense and action-oriented when one starts attacking the players. Crashing sounds are common as well, as they throw cars and chunks of landscape as well as rip apart doors.

Strengths: Where to begin? The Tank is the single toughest Infected in the game. They can absorb an unholy amount of damage, forcing the entire team to work together to kill one. They can rip off chunks of the environment to hurl at the players, and will also throw cars. They can tear through certain parts of the environment, like pillars and barricades. Not even safehouse doors can stand up to a Tank. A player that is hit by a Tank will be flung, heavily damaged, and will be stunned on landing, unable to move.

Weaknesses: A Tank can only take on a single survivor at a time, where all survivors can unload on them. They can't bash through obstacles, so walls and certain environmental objects will hinder them. They have to climb up like any normal Infected, as they can't leap. Any long-range attack can be easily dodged. They have a huge profile, which makes it almost impossible to miss them.

Special: Tanks are big, dumb brutes, so use this to your advantage. They take time to find a target, and will get slowed down by environmental obstacles. With one person acting as bait, a Tank can be taken out extremely quickly as long as anybody, bait included, has a semi-automatic shotgun. Tanks are also highly flammable, and will burn until they die.
Witch

Appearance: Almost-naked, panting, gaunt, disturbing-looking woman with huge claws. Always has a red light on them, no matter where they may be in a level. Witches are the exact opposite of Tanks in appearance, being thin, small, and low-profile.

Sounds: There is no creepier ambience in Left 4 Dead than that which surrounds a Witch. The Witch cries and sobs as long as it has been undisturbed, and the music becomes very creepy and disturbing as one gets closer and closer to a Witch. When they have been startled, it screams and chases down the offending survivor, letting loose random cries.

Strengths: The Witch is the second-toughest Infected, next to the Tank. They also deal the most damage out of any Infected, as they are able to incapacitate (Easy through Advanced) or kill (Expert) any survivor with a single strike. They have great hearing, and any gunshots or explosions nearby will startle them, causing them to chase down the survivor that startled them.

Weaknesses: The Witch will only chase down a single survivor before running away. The survivor which is closest and dealing the most damage is a target, but the one that startled the Witch will usually be the one that ends up taking the hit if it gets close enough. They're fast, but are slowed down a lot by doors and other obstacles. They can be sneaked by if the survivors turn off flashlights and walk instead of run.

Special: The Witch is the only special Infected that can't be controlled by a player, for obvious reasons. A survivor with a semi-automatic shotgun can sneak up on a Witch (right next to one, in fact), point their gun at her, and unload a full clip into her before she can fully respond. This is the easiest way to take out Witches that are blocking essential paths, such as ones that are in subway cars in the second level of No Mercy.