While doing research to find out if epilepsy runs in my family, I spoke to my mother about seizures. She claimed she had them. I did a bit of research on the net and discovered it's sleep paralysis (which is not a seizure). Dervak's done a pretty good job, so here's some info I can contribute:

Causes: I've reworked this since I've learned a bit more. Sleep paralysis is not a disorder unless it's isolated (i.e. doesn't occur with other symptoms, most notably excessive daytime sleepiness or cataplexy)). 6% of the US population will experience it at least once, but if it is recurrant, it is often symptomatic of a sleep disorder, most notably narcolepsy ... so bear that in mind before you say "I wish I could have out of body experiences." (Also note that incidence of narcolepsy is about 1/1600 - in other words, of the 6% of Americans who will experience sleep paralysis, only about one out of every hundred of them has narcolepsy). The causes are unknown, but seem to have something to do with interference of hypocretin(sp?) aka orexin (sp?) levels in the sleep mechanisms of your brain. Sleep paralysis is atonia (loss of muscle tone that prevents you from acting out your dreams) typical of REM sleep, so sleep paralysis is often accompanied by hypnagogic hallucinations. In other words, your body goes to sleep before your mind and ears (and often eyes) do.

What to do if it happens to you: It's normally not a problem if it only happens once or twice. If it happens often, it's usually not even a problem then. You may find it happens more often if you work nights or stay up late. In these cases, just get a more normal schedule. However, you should bear in mind that it is a sign that your sleep cycles are disrupted, and it can cause permanent damage, so you should get your sleep cycle back on track. However, since I am not a doctor, don't take my word for it (though family practice doctors tend to not to be familiar w/ it). For good research material, /msg me and I'll add it to this writeup.

Symptoms: These are my personal symptoms and it should be apparant why I don't like it. I hear a buzzing/ringing/screeching sound that's pretty indescribable -- it's like what the pins and needles you feel when your foot goes to sleep would sound like. Speaking of sleeping feet, feeling that same feeling over your entire body is quite normal. I sometimes experience a sharp pain in my lower back and/or kidneys (or thereabouts). Feelings of dread are very common, as are those of being accompanied or watched by a "presence" (and also often happens in my case), and hallucinations are common as well (remember, folks, this is REM -- everybody hallucinates in REM). I've had exactly one OBE. I'm not sure how common it is, but I've also noticed if motion occurs, it happens very slowly -- like a movie playing one frame every 2-3 seconds.

Prevention: Certain antidepressants can help. <updated>I am actually not sure if MAOIs cause or prevent. Tricyclic antidepressants and SSRIs do help if it is related to cataplexy (i.e. you have narcolepsy), and as I understand it, so do MAOIs, but this latter class of antidepressants is no longer widely used. The reason why they help is that sleep paralysis is a malfuntion of the normal REM mechanism (certain events in your brain happen out of order: suppress REM with happy pills and the SP goes away). As I understand it, the REM suppression occurs whether or not you actually have narcolepsy. But talk to your doctor if you decide you want to change your medication </updated>

The easiest non-drug way to prevent seems to be to change your sleeping position. In the forums I've visited, most people have found varying their positions lessens or eliminates sleep paralysis. Most people (I've read about) experience it on their backs. Lying on the stomach is less common, and I haven't seen anybody complain about sleeping on their side. (Incidentally, that's exactly the case w/ me -- I never sleep on my back anymore, and I didn't even learn this consciously!)

How to wake: According to some studies I read, the best way is to move your eyes and try to flutter your eyelids. Trying to move your fingers apparantly also helps. According to most people with the disorder I've read in various forums, attempting to shout and rolling your shoulders seems to work best. I do the latter. Remember to vary your sleeping position when you come out of it, otherwise it seems to generally (in my case and most others I've read) recur within a very short amount of time (a couple of seconds to a couple of minutes)!

The supernatural stuff: I was once attacked by some etheral creature (I hate to say demon). I was once attacked by a lion. I was once thrown from my bed by an invisible force (I woke up still in bed). Other people report being stabbed, abducted by aliens, burned, and all other manner of torment (and mind you unlike normal REM, you are conscious -- this hurts!). It's not too easy to get around this (for me), but apparantly it can be done once you realize it's not real (of course, you're in REM mode, so this can be difficult!). It's just a dream. Relax, and if it's not bad, you can try to go with it (OBE), or if you don't enjoy it, try to get out using the techniques above. It is also (apparantly) possible to self-induce this state (even without a history of it) through meditation, if you're into that kinda thing (of course, I didn't choose this -- it chose me). One buddy of mine tried medatation and quit because of this! ("I don't like it! A thousand fingers touching me!")

Two flavors: Hypnagogic occurs before you fall asleep (I get this one most often). Hypnopompic occurs after you wake up -- the dream within a dream. I've had only one of this variety, though its actually the more common variety. I believe these terms technically refer to the accompanying hallucinations rather than to sleep paralysis, but I've often heard them used to describe sleep paralysis as well.