Here are the three main theories:

Something from Outer Space
(Solar flares, meteorites, comet dust, something nasty that the astronauts brought back, etc.)

Science Gone Berzerk
(Chemicals, electricity, radiation, etc.)

Magic
(Contingency spells, ghostly vengeance, standing-room-only in Hell, some jack-ass playing around with the Necronomicon, etc.)

In addition, a common feature of many zombie stories is that the zombies, regardless of how they were re-animated, are contagious: a zombie bite transmits a deadly virus which kills you within hours, and then you, too, will become a zombie. (And of course, we just live to bite people. Right. We'll get to that.) These epidemics are a special case; but for now, let's look at just one instance of re-animation.

True zombification is a result of very specific electro-cellular activity within the corpse itself. All of the above can, under the right circumstances, initiate re-animation. For now, let's concentrate on the effect itself, rather than the causes of it.

It all comes down to energy. Stay with me here.

Life after Life
You live people draw your physical life energy from your the energy of your native material plane. That plane has an energy frequency, like a radio station. Your awareness is so tuned into that frequency that it's difficult, if not impossible, for you to tap into the frequencies of other planes. But you can imagine them, and that's important -- your personal frequency is a combination of your material plane's frequency and the frequency of the thoughts that your mind is constantly receiving and projecting. Every plane of existence has its own frequency, and the collective energy of that plane vibrates at that frequency. (Thus, if enough people project thoughts of a particular frequency, they can, eventually, change the overall frequency of their material plane.)

At the subatomic level, all the atoms of your body are really nothing more than energy. Your thoughts, as brain waves, also have a measurable energy. During your lifetime, the bulk of your thoughts (or "soul") are centered around your perceptive center -- your material body. When that machine finally wears down or is broken beyond repair, it stops working and dies. But since the Law of Conservation states that energy can neither be created nor destroyed, but only change form, the thought energy is still there, and needs to find a new perceptual center. The "soul energy" finds itself on a new plane -- the frequency of thought energy on this material world is the frequency of matter there. Your thought energy becomes the material energy of your new body, and you find yourself in an environment that's attuned to the total frequency of that body.

That's your afterlife, so be careful. You may be what you eat here, but you are what you think there. Ok, whatever, enough moralizing.

The Circuit
The first step in re-animation is to jump-start the body's cells with a fresh influx of energy. Now, hang on -- I see you all grabbing your tasers and getting ready to head on over to the morgue. Sit back down; did you really think it was going to be that simple?

Wow, you people really want to try this, don't you? Allllll right...I have a feeling I'm going to regret this, but here goes...you nutballs click here. For the rest of you who want nothing more than a synopsis, just stay on this page.

As I was saying..."life" is usually a progression of awareness in one direction. You're born, you process the energy of your material world throughout your life, and then you take that energy with you when you die. In fact, that energy is you, once your native-plane body deteriorates. Like Hawking's "Arrow of Time", the process -- supposedly -- only goes one way. But we're not talking about time here. (I'll leave that to Jymi.) We're talking about an energy current. And as we all know, energy currents, even those which are so highly evolved that they're aware of themselves, are reversible.

In zombification, the energy current is pushed back the way it came so that it completes a circuit -- from the source on the material plane of origin*, to the "afterlife" in which the current resides, and back around to the source. Note that the energy does not leave the "afterlife" and drain back to the material plane level -- a zombie is a walking (or shambling) transmitter / receiver of his or her own personal energy frequency, connected to both planes.

What happens to the material body that was created in the "afterlife"? Well, how the hell would I know? My body, and my consciousness, are here, not there, as far as I know. But I'll tell you one thing: I don't remember my dreams ever being as vivid when I was alive as they are now that I'm undead. Maybe it's like a summer home.

The zombie's level of conscious awareness of both of his or her planes-of-residence depends on the individual, and how much time he or she spent in the afterlife. Most of us tend to center ourselves wherever our native-plane body is, just like you do. Some, though, never quite come all the way back. In any case, once the physical rehabilitation is complete and the psychological development reaches the point where the personality has fully returned, some of us can do some pretty weird things. Which brings us to...

Magic Zombies: Abra-Cadaver!
You get your life energy from your native material plane. A zombie's "life" energy comes from the plane to which their soul migrated when the material body died. Some of those planes can be fairly powerful, and that which you'd call "miracles" happen there all the time -- it all depends on the nature of the energy frequency of the thoughts and beliefs held by the zombie while he or she was alive. So when the energy returns to the material plane and reinhabits the body, it can sometimes bring with it a host of new abilities and/or knowledge that seem incredible or even supernatural to those who are still firmly rooted in the material plane.

Before you start rolling your eyes, remember that once upon a time, even electricity was considered magical by those who didn't understand it. If you travelled back to a time before humans harnessed electricity, that wouldn't take away your knowledge of the power. If you then built a simple electric generator and used it to light your cave, the rest of the people would consider you the greatest magician ever, erect statues of you, and probably offer you a virgin or two (either that or they'd think you're a demon, and would try to poke you to death with sticks). Us "magic zombies" are simply carrying the knowledge and skills that we learned in the "between-time".

I can do some neat stuff: mostly simple energy manipulation (telekinesis, pyrokinesis, that kind of thing), and suggestive hypnosis (useful for subduing my supper, but it really comes in handy when I want to get out of a speeding ticket). Since my energy circuit is under control, I can also use that connection to create another zombie. Nobody's offered me any virgins yet, though I have been poked at with sticks now and then. Please tell people not to do that; it's annoying, and it hurts. (Yes! Zombies feel pain! See the next section, "Dealing With It.")

Take a Bite and Pass It On: Cannibalism and Infection
What is it about eating live folks?

Well, for one thing, you're delicious. Raw or cooked, but preferably still kicking.

Ok, ok, I'm sorry. We do have to consume living energy, but that doesn't mean that all of us like it. In fact, that's the one thing about being a zombie that I really, really hate. If it seems like I'm making light of it, that's just because whenever I think about it too seriously I want to throw myself in front of a train or something. (Not that it would do any good.) But you've been patient enough to come this far with me, so you deserve an explanation. Maybe you'd like some lunch while you read this: how about a nice cow sandwich, or a salad made of fresh, helpless vegetables? Mmmmm. Life.

You'll find more information on the dietary needs of the undead in the next section, "Dealing With It." But since this section is all about making new zombies, I'm sure you're asking this question:

Why is it catching?

In damn near every zombie movie that's ever been made, if a zombie bites you or otherwise injures you by breaking your skin with a part of its body, no matter how large or small the wound, you're dead -- and you come back. What up wit' dat?

Though most of the clever scientists try to pass it off as some kind of virus, it's not. As you'll read in the next section, zombies do need to eat live food, and a Cerebellic-level zombie is a storehouse of powerful, random energy surges. When one of these guys bites a live human, or even scratches them hard enough to draw blood -- anything that involves the excellent conductive capacity of bodily fluids -- it creates a path for this other-planar energy. Even if the zombie is pulled off its victim, a permanent conduit has been created between the living body and the other plane. At least, it's permanent as far as I know. I have yet to find a way to block the circuit in Cellular and Cerebellic zombies, short of woodchipper-level dismemberment or cremation.

(Note that this only applies to zombies who have yet to reach the equilibrium of the Cerebric level. By that point, the circuit has stabilized and they've learned to contain and control it. These advanced zombies are only contagious if they want to be.)

A live body can't handle the powerful energy flow from other dimensions, especially those which don't resonate with their own "soul" energy. But the connection from that body to the other planes has been established, and their spirit (or thought-energy, if you prefer) begins to seek out the planar energy with which it does resonate, kind of like how the molecules of fluids of varying densities in one beaker will form layers. The person will eventually succumb to the energy overload (usually within a week or two for a non-fatal wound, or right away, for the real gut-wrenchers -- loss of blood can play a big part in bodily death, you know). If their body is reasonably intact, nature takes its course, and they will shortly return as a newborn zombie, bearing the energy of their own "afterlife" plane. Be nice to them. They've had a hard trip.

*Phrases like "material plane of origin" and "native-plane body" refer to the plane of existence on which the re-animated body in question was born, lived and died. I'm not even going to get into reincarnation and multiple lives lived in different time periods on one plane. Not until Medoc brings me some more of those funny little plants to smoke.