A cult founded by science fiction writer L. Ron Hubbard. There core beliefs are as follows:
This area of the galaxy was once ruled by an alien warlord named Xenu. 65 million years ago his empire was overpopulated horribly, so to combat the overpopulation he had trillions of citizens called in for psychiatric evalutations and tax audits, where they were given paralytic drugs, loaded onto space planes, and flown to Planet Teegeeack. Once on this planet, their bodies were stacked in huge piles around volcanoes, while atom bombs were dropped into these volcanos, instantly killing trillions of galactic citizens.
To keep these people from reincarnating in the rest of the galaxy, Xenu set up a special trap. Powerful force-fields kept their souls (called "Thetans" in Scientology jargon) from escaping, and these thetans were then shown special holographic movies about the various lies they are supposed to believe, with characters like Jesus, Mohammed and Buddha. Over time, these thetans began to believe Xenu's lies about false gods, and began to reincarnate on Teegeeack (but the thetans called this world "Earth"). When intelligent life appeared on Teegeeack, thousands or millions of "Thetans" cling to every native soul (er, "thetan") and fill it's head with nonsensical thoughs and irrational beliefs.
The key idea of Scientology is to avoid tax officials and psychiatrists at all costs, while paying the church a large amount of money for special "counseling" (called "auditing" in their jargon) to help you reach a special enlightened state called "Clear" where you telepathically communicate with these "thetans" clinging to you and force them out of your body, and rid yourself of evil Xenu's influence.
(By the way, most Scientologists don't know this, like the Mystery Cults of the Greco-Roman world, Scientology doles it's doctrine out in tiny doses to it's new recruits, and tells the public next to nothing about what it believes. You don't hear that whole story until you reach a special rank in their group called "OT3" for "Operating Thetan 3". Their doctrine holds that if this got out, the Thetans in people would revolt at hearing the truth and people would get sick and die possibly just by hearing this story, which of course leads people who aren't Scientologists who hear this story to tell it to others and watch them not die or get ill from it.)
The Church of Scientology takes advantage of the gullibility of the American people to make a tidy profit.