Having contracted some sort of virus, these corpses move like living things. Their only apparent drive is too propagate the virus. Recognizable at a distance primarily by their bloodshot eyes, though particularly old zombies do begin to pale.
Zombies seem to have suffered none of the slowing that effects Ghouls. As well, while they will chase Ghouls they give up the attack after a few bites (though they may have already severely injured the Ghoul by that point).
Zombies are highly resistant to attacks against most of their body. A gut shot will kill a zombie eventually, but it does nothing to slow its attack. The safest course of action is to shoot ‘em in the head.