I used to experience sleep paralysis, though I've out grown it since I've grown older. You are awake, and can even see if you open your eyes, but it is impossible to move, talk, and sometimes difficult to breathe. When I was younger this simply terrified me. There was nothing I wanted more than to wake up, and I couldn't wake myself. I would think to myself "OK, on three, 1, 2, 3, GO!" and nothing would happen. After a few minutes I would come out of it though. It usually only happened to me when I was dozing off or taking a lite nap.
From what I've heard, there is a part of your brain that essentially paralyzes your body in your sleep so that you don't move around too much. What happens is that this part of your brain doesn't "turn off" properly, and thus you stay paralyzed even though you are awake.
Ryan