Some of you might wonder exactly what classifies something as a zombie. Well, people's opinions vary, but the most common concensus is that a zombie is any re-animated corpse. You can take this a bit further and say that any re-animated once-dead body part is considered (a) zombie.

Given this, there are conceivably many different types of zombies.

toxic zombies

Brought back by radiation or chemicals; usually flesh-eating
This is by far the most common type of zombie portrayed in films. Quite often the government has something to do with the 'birth' of these. Watch out for Army trucks with barrels on the back going through your town!
Movie examples: Night of the Living Dead series, Re-Animator, Redneck Zombies

demon zombies

Re-animated by demon, devil or other evil spirit
Movie examples: Evil Dead trilogy

voodoo/black magick zombies

Brought back by a houngan or bokor (a voodoo master)
Movie examples: White Zombie

diseased zombies

Vague type spread by infection, such as by the bite of the Sumatran rat monkey
Movie & Game examples: Dead Alive, Wes Craven Presents Mind Ripper, Resident Evil series

unsettled zombies

Return because of some unfinished business of some sort, usually revenge or love
Movie examples: My Boyfriend's Back, Creepshow's water zombies, Uncle Sam

alien zombies

Brought back by aliens; usually to serve them as an aid in taking over the Earth
Movie examples: Alien Dead

techno zombies

Re-animated by technology; implanting a computer chip in the brain, etc.
Movie examples: Deadly Friend

electric zombies

Brought back by a shock from electricity of some sort
Movie examples: Dead Heat

Find out what the reference 'books' have to say about zombies.

Dictionary.com

InfoPlease.com

Some psychologists use the word zombie to describe a mental disorder of sorts.

Zombies on the Web

Bringsjord's Zombie Papers

What do you think? Let me know and I'll post your comments here!