Finally, something that I can add my expertise to.
There are two kinds of zombies, the Voodoo (Voodun or Hoodoo) zombies, and the undead zombies. Voodoo zombies are created by first drugging the victim with a mild to moderate sedative, which puts the victim into a comatose state. The victim is then buried alive, where the lack of oxygen and the effects of the sedative serve to severly damage parts of the brain. When the zombie is exhumed, he no longer has any of the intelligence he did as a human, and will listen to what the witchdoctor tells him to do, through a little magic and alot of authoritative speaking.
It is less clear what would cause an undead zombie, but most think of it as some sort of viral infection that centers in the brain. The affected zombies are in fact dead, their heart stops, pain receptors shut down. They retain small muscular control, allowing them to shuffle toward their victims, and by instinct they draw air in and out of their lungs, despite their lack of a working cardio-respatory system, in order to make their distinctive moan. The virus keeps the brain, parts of it anyway, alive and allows the body to die slowly. An undead zombie is never as strong as when it was first created, since it's body does not repair itself, where as a Voodoo zombie is still alive, and thusly will eat, sleep, etc, and its body will repair the minute tears in the muscles caused by movement.
This is getting long (getting? it already is) but I hope it helped.
