It used to be called "Old Hag", which is where your friends might be joking around. People used to think it was the spirit of an old hag sat on your chest, preventing you from waking / moving, etc:
http://paranormal.about.com/library/weekly/aa1 12000a.htm
As ice maiden has pointed out, it is actually sleep paralysis. It is something to do with when you reach a certain period in your sleep, your brain releases a hormone which paralyses your body this prevents your from waking during a dream, i.e. if you dreamt you were walking along and chatting with a friend, then if your brain did not release this chemical you actually would be walking along chatting aloud. Sometimes, particularly if you are stressed or overtired, your brain can "paralyse" your body, thus putting your body to sleep, even though your brain is awake!
http://en.wikipedia.org/wiki/Sleep_paralysis
It's nothing to be frightened of, try to relax before you go to bed. The first time this happened to me I was petrified, and it happened to me a few weeks ago but now I know what it is I can sort of control it because whilst my body is paralysed I can keep telling myself that I AM AWAKE! It sounds mental, but if you shout in your head to yourself then you do wake up and can go back into a normal sleep. If that makes sense. Hope that helps x x