Cadaverus Apparent:
These are your everyday Voodoo zombies. Despite what some of the movies featuring these jolly characters may imply, a Voodoo zombie is not a walking corpse. I realize that the Voodoo culture gave us the term zombi in the first place, but those people are just as alive as you and me...well, you, maybe. The bokors (or witch doctors) poison their targets using a solution which contains, as one of its active ingredients, tetrodotoxin -- a secretion of the fugu, or "puffer fish".

(And no, I'm not going to give you the recipe. You sick little monkey.)

This solution causes the body's functions to slow down to the point where the person appears dead. The brain, however, is still active, and very much aware of what's going on around them. They can hear the doctor make the pronouncement of death; they can hear their loved ones crying; they can listen in on their own funeral (and who wouldn't go for that, if given the chance?!) Finally, they hear the clods of earth as the gravediggers cover their coffin. When the poison wears off (if it wears off -- depending on their constitution, they just might die from it) the bokor comes around and digs them up. Imagine the effect this has on someone who comes from a culture where zombification is taken for granted. They're still woozy from the poison, and they believe themselves to be dead and completely subjected to the will of the bokor. Poor guys.
See: The Serpent and the Rainbow; White Zombie

Cadaverus Habitus:
Now we're talking corpses, but these bodies are more like puppets, controlled by another entity. They're not walking around under their own power. The possessor may have taken up residence within the body, or it could be using some kind of paranormal ability, such as telekinesis, to animate the body. Though dead, this is not a true zombie.
See: Evil Dead (any of 'em)

Cadaverus Bullshittus:
See: Any of those films from the 50s that say they have zombies in them, but it only turns out that the characters have been zapped with some kind of mind-control thingy. Ho-hum.

Cadaverus Absolutely-Notus:
I don't care what you think, the people in the movie 28 Days Later are NOT zombies. They're live folks with a disease. It's stiIl a good flick, but I swear, the next time I hear someone refer to those guys as "zombies", I'm going to give them such a bite.
See (for yourself): 28 Days Later

Now that that's out of the way, let's move on to the the true zombie.

Cadaverus Verus:
Dead man walkin'!
The true zombie is a body that was once alive, has been dead (for any amount of time), and is now re-animated, moving under its own power. They exhibit varying degrees of decision-making behavior, personality, and intelligence.

There are four levels of re-animation:

1. Cellular -- We all start here. Before the body can get up and move around, the cells need energy moving through them. This is the most basic form of true zombification: get the cells going, and you've got a baby zombie.

(Awww, iz'nneeecuuuute! Woochie-woochie-woochie...)

These guys can barely stand up. The most you'll get out of them is maybe a low groan, but I wouldn't expect too much. They don't even realize that they're hungry; they're just kind of...there, like a bad hangover in a 7:00 am trigonometry exam. All of the re-animated cells are still working independently, but the brain and the central nervous system haven't yet re-established the connections necessary for coordination and balance, much less beating down doors and chasing after a live victim. Movies featuring cellular zombies will usually imply that they have some kind of agenda, such as attacking stupid live folks who bumble into places where they don't belong. Good storytelling; poor fact-checking. In the real world, these zombies don't take orders, don't represent the Hellish Powers, and won't try to gouge your eyes out. They're just trying not to fall over.

Good morning, sunshine! A zombie in the cellular stage.

See: Seven Doors of Death (Not a great example, since most of the zombies don't appear until the end, and then they're attacking. But look at the zombies' movements, not their behavior. Realistically, cellular-level zombies just aren't very horrific.)

2. Cerebellic -- Now the cerebellum and the hypothalamus are back on-line, and the nervous system is re-forging those important connections for voluntary muscle control. The zombie can move around without stumbling too much, and is aware of, and will interact with, its surroundings. The intelligence still leaves something to be desired, but the zombie has basic goals and a survival instinct. There might even be some stirrings of a return of the original personality, since this level and the next (Cerebric) have a lot of overlapping phases as the neurons which control basic functions will often begin to randomly jump-start the ones which control the higher functions. Even the least intelligent of these guys are capable of dog-like thought.

This stage is also characterized by the "contagious" nature of the zombie, the cause of all those epidemics that are so much fun. However, the scientists are barking up the wrong gravestone here -- it's not a virus; it's an energy surge. We'll cover this in detail in the "Zombification" section.

Much like the physical development process of living children, a zombie's coordination and balance skills will improve with time and practice. And like children, the rate of development depends on the individual and his or her physical and neurological talents and limitations. If someone was clumsy in life, they're not going to make the gymnastics team once they're undead. We have to work with what we've already got. The newer films featuring the "fast zombies" are simply concentrating only on those individuals who have -- very quickly -- passed the stumbling-around phase. Again, makes for a great film, but not too realistic.
See: Any of the Romero films, especially the character "Bub" in Day of the Dead, and the lead zombies in Land of the Dead for examples of the Cerebellic/Cerebric overlap. For "fast zombies", check out Return of the Living Dead and the 2004 remake of Day of the Dead.

3. Cerebric -- All of the neurons have returned to service. Emotions, consciousness and memory are all fully functional.

This zombie is fully aware of its surroundings and circumstances, capable of higher reasoning, and the personality is making a comeback. Of course, discovering that you're a zombie tends to alter your personality a little bit...
See: Re-Animator (and its various sequels), Frankenstein, and even though the movie sucks, that zombie chick in Return of the Living Dead III.

4. Cadaverus Spiritus -- Hey, that's me!
As the personality comes back, the soul re-awakens, too. Having spent some time on other planes before returning, the soul may have gained some otherworldly knowledge and connections, including what you might call "magical" powers.

A-hem. I sense that some of my readers have raised a collective eyebrow here, and are patiently awaiting an explanation. You'll find that in the next section, "Zombification."