Wikipedia is descidedly biased on the matter. So am I though.
It's a church, founded by L. Ron Hubbard, a science fiction writer.
They essentially believe that some alien race imprisoned another alien race in a volcano, wehre they died, and that they were reincarnated into us or something. It's rather complicated. There's a book called 'dienetics' that is essentially their bible. Read it for more info.
The basics: Xenu is the bad guy; you must pay money to enter the religion; you are not allowed to seek pharmacological treatment for what ails you as it is not part of their religion (this includes medications for depression, ADHD, etc.); phychology and psychiatry are considered wrong by them, described as a 'crackpot' science; mothers cannot cry or make any noise while giving birth, they believe this scares the child and scars it as well... no idea where that fits in; and that Hubbard created the religion on a bet with some friends.