This is the most comprehensive description of Scientology beliefs I have come across. I got it off the Tom Cruise Chalkboard. You should check it out if you want to learn more - there's an ex-scientologist there who was in the cult for 30 years.
"What are BTs and Clusters, you ask? Well, 75 million years ago, an evil galactic ruler named Xenu was faced with an overpopulation problem. So, with the help of Psychiatrists (yes, according to $cientology, Psychiatrists have always existed and have the intention of destroying mankind) he drugged and paralyzed billions of his people and piled their bodies at the bases of volcanos, right here on Earth (then it was called Teegeeack). Then, he exploded the volcanos with atom bombs (I guess they had those back then too).
After the bodies were destroyed by the explosions, Xenu captured their souls in electronic beams. He then 'implanted' them with 'false data' about reality -- oh, things like Christ, Budda, other dieties, etc. After the implanting, the souls thought they were one and the same being, and began to 'cluster' together into groups of souls.
Now, everytime someone is born on earth, cluster of these souls or 'body thetans' (BTs) cling to the person. $cientology can help! Only certified $cientology 'tech' can rid one of these harmful BTs and clusters."
For more information, go to
www.xenu.net
or
http://www.ixplosive.com/cgi-bin/yabb/YaBB.cgi
And, umm, would you join this cult...?