That's a legitimate question! Part of the whole problem in this field of study is that there's no widespread agreement on even such basic things; everyone has their own idea and no sound research has been carried out to establish what it is we are even talking about.

I could talk at length on the subject but I couldn't do it any better than Gopi already has; you should check out "The Real Nature of Mystical Experience"

http://www.amazon.com/Real-Nature-Mystical-Experience-ebook/...