Acording to the church of scientology, body thetans are the souls of murdered people who where brought to this planet by Xenu.
Xenu, the Emperor of The Galactic Federation, is said to have taken all of the overpopulation in this sector of the galaxy and brought them to Earth to be place under volcanoes and blown up with H-bombs. The souls of all of these murderd people then roamed the Earth and started colliding with each other until they became only a few beings of many souls. These beings then infest the bodys of humans. Scientologists believe that these body thetans are the reason for all of their bad memories and must be excommunicated telepathically. The process of the excommunication of the body thetans can only be done after a Scientologist has reached a point where they are free from all distractions of the mind, a point which they call "clear". This is one of the highest levels acheived in the "Church" of Scientology.
The previous information has been gathered from various web sites, but also from general conversations with people that I work with. I work in a company that seems to have been founded by Scientologist, and even since our merger with Mindspring still holds many of their origonal values. Because of this many of the employees have taken an interest in the "Church" of Scientology and therfore are a wealth of information. | 
