The Charger

The Charger is a charging Special Infected that knocks everything in its way, it is a new Special Infected to appear in Left 4 Dead 2 and will be playable in Versus mode. It makes a donkey-like groaning sounds as seen in the first teaser while running toward Coach. It is described as serving to split up Survivors who are grouped up in a small place. While it matches the Survivors' speed, it can charge toward the Survivors and become three times faster. Unfortunately, this will lead to a slow stop and difficulty at turning. It's not as powerful as the Tank, but can still take a beating and do half of its damage by charging. It is also less maneuverable, making it easier to dodge as it charges around.

It does damage per hit when its picking and grabbing Survivors up in a "pinned-style" much like the Hunter and Smoker, and then smashing them repeatedly into the ground, but it does much more damage than the other Special Infected such as the Hunter and Smoker, possibly doing around 10-15 damage per hit on easy mode.

In terms of appearance, the Charger is a large Infected, roughly the size of the Tank. Unlike the Tank however, only one of its arms have been mutated while the other is small and presumably useless. It also sports blue-overalls, meaning one could assume the Charger was once a farmer, especially considering the Southern setting. Also, unlike the Tank, the Charger does not use its arms for movement, as its legs are still capable of supporting it. The Charger seems to prefer using the arm as a shield. Like The Tank, though, it charges through crowds of Common Infected to get to the Survivors.

Like most other Special Infected, the Charger has a distinctive sound. It does not shake the ground as the Tank does, but itself does seem to be loud, growling and moaning at enemies distinctively.

Not much else is known about the Charger, however, if the original Left 4 Dead campaigns are included in the sequel, it is questionable if they will be updated to include the Charger, as it seems to be created specifically for the theme of the new campaigns.

Extent of Mutations
The Charger has been heavily mutated by a strain of the infection. It appears to be halfway into the transformation to a Tank. While not quite as hulking as a Tank, it has an enlarged torso and right arm and an abnormally shaped head. Its left arm is shrivelled and hangs oddly off the side of its hunched back. It wears a pair of blue overalls that have slightly burst due to its malformations. While pounding a Survivor, the Charger's useless arm flops around comically. Possibly due to its choice of attack, the Charger appears severely damaged; its overgrown arm possesses a big festering wound, and the creature's scalp and face appear smashed or caved in.

Even though it does seem like the Charger is half way into a Tank, at the same time it also seems to be turning into an Infected all of its own. The concept art of the present Charger shows that its left leg is much larger in size than the other meaning that he might be turning into a bipedal Infected. However, unlike the Tank - who moves in the same fashion as a gorilla - the Charger could get around much like a rhino does. He will continue charging at the Survivors but he will not be able to throw slabs of cement like the Tank.

Trivia

 * Originally, through concept art the Charger was going to be either a large zombie with a armored upper skull yet tiny "T-rex arms" or a "tripod" dog like animal.
 * The visible mutations of the Charger seem similar to that of an extreme case of a disease known as Elephantiasis.
 * The Charger bears a striking resemblance to Nemesis and Bandersnatch from the Resident Evil franchise.
 * According to PC Gamer Magazine, the Charger (with super-scientific calculations) has three times the speed of Common Infected whilst charging, twice the health of a Smoker, deals half the damage of a Tank (unclear if it is in one strike), is as tall as two Assault Rifles put together from butt to barrel and apparently dresses better than the Witch to the power of Boomer.
 * Sounds used by the Charger at Comic Con are re-used ambient sounds from various sections of Ravenholm in Half-Life 2. In addition, the Charger's roars and grunts seem to be remixed or altered versions of Half-Life 2's Poison Headcrab Zombie.