It's a religion founded by a science fiction writer who degenerated into lunacy as he surrounded himself with people who agreed with his every word. Many of his acquaintances from before his Scientology days say that he was always prone to inventing wild stories about his past and swearing they were true even though they were plainly impossible.

On the outside, Scientology recruits people by promoting itself as a therapy. They claim that Scientology is not incompatible with any other religion. Once you're recruited you go into a tiered system. You learn more about Scientology as you move up in ranks. To move up the ranks you have to pay for training sessions. If you can't afford the training, you can pay for them by working for the Church of Scientology. If you don't complete the training sessions to their satisfaction you have to pay for the course again if you want to move up. Progression requires money and compliance to Scientology.

 The early training sessions are sort of self-helpish training for being better at life sorts of things. As you move up, you learn more about Scientology and its beliefs and you're taught that as you move up you will become a better person. Later on they start telling people that inside everyone there's an ominipotent spirit called a thetan and that they will be able to free the true powers of their thetan if they keep moving up.

Pay enough and you get to find out why your thetan is stuck in your loser body. It turns out that a long time ago there was a space dictator named Xenu who wanted to get rid of the excess population in his space empire. So he packed up all of the people he wanted to get rid of and brought them to Earth. On Earth he brainwashed them to believe in various things which would trap their immortal thetans on Earth. Hubbard claims that the images from the brainwashing have formed the basis of all human religions other than Scientology. Then Xenu put hydrogen bombs in volcanoes and blew up all of the people. Rather than go and be free the traumautized thetans gathered together in clumps and today there are lots of thetans sticking to your body that prevent your thetan from being the stud he's capable of being. But fortunately Scientology can help you rid yourself of these pesky thetans.

 L. Ron Hubbard also wrote lots of other interesting things from tips on childcare to his idea that science fiction is just people remembering their trillions of years worth of past lives. Oh, and if you're concerned about the possibility of Xenu coming back, don't worry. He was later overthrown and trapped in a stasis field where he remains to this day.

Wikipedia is a good source for more information.