I think Howie's thread about it is the best introduction. It is certainly better than anything I could write right now,

http://dreamviews.com/community/showthread.php?t=57097

In this context, it is when you wake prematurely and find your body is still physically asleep. Your mind wakes before your body does. Your body feels paralyzed, and there are some common hallucinations that people report. You are only half awake, so you dream right over what you think is reality. It is an amazing state of consciousness and a huge contributor to countless legends throughout human history.