The Charger

The Charger is a charging Special Infected that knocks everything in its way. It is a new Special Infected 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, making it 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 after reaching a wall or other viable hard surface (since the update), 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. This delivers 22 damage per hit on normal difficulty. When pinned by the Charger, Common Infected can still do damage to the Survivor like the Smoker.

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.

While charging, the Charger will run like a Tank, using its mutated arm to walk in a "monkey" style. It will run faster than a Survivor with full health and it will send the one he hits flying in a similar way to the Tank (this attack does about 8 damage on easy). Then, it grabs and slams the closest Survivor to it (this attack does 10 damage on easy). After that, it slams the Survivor in a pinned like style until the Survivor is dead or the Charger is killed. This attack does 15 damage per slam on easy.

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 shriveled 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; the creature's scalp and face appear smashed or caved in.

Even though it seems 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 intro scene of Left 4 Dead 2 shows that its left leg is much larger in size than the other, meaning its legs are starting to lose support on it, similar to the Tank, and that the right leg is under much more weight because of its large, mutated arm.

When it charges, it uses its right arm to run like a Tank, furthering the similarities between the two.

Behind the Scenes
Before the Charger's design was finalized, it used a reskinned Hunter model so that the team could test out how it played while the art team was designing it.

Trivia
http://www.hl2world.com/bbs/3-vt50304.html?start=30
 * Concept art shows that the developers had planned for the Charger to be either a large zombie with an armored upper skull yet tiny 'T-rex arms' or a 'tripod' dog-like animal.
 * Originally, the Charger would stop immediately when it had grabbed the closest Survivor; now, it continues charging until it hits a wall, then starts slamming. However, this is most likely the developers testing different ways the Charger can attack, as this would unbalance gameplay and render the Smoker and the Jockey useless. If given this ability, he would be able to pull a Survivor extremely far from its team or even kill them by dropping them off a cliff.
 * The Charger bears a striking resemblance to Bandersnatch from the Resident Evil franchise. It also appears similar to the build of the Flood Tank Form from the Halo series.
 * 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.
 * Charger is also a name of an alien enemy cut from Half-Life.
 * The Charger's model appears to have been changed again, now more closely resembling it's skeletal-faced concept art. It's ramming arm now seems to have become not just bigger, but rock-hard aswell. It can be seen here: