As usual: http://en.wikipedia.org/wiki/Scientology
BONUS: http://www.youtube.com/watch?v=rD9bCdHqU 3s
The last one is interesting. He tells you how he got roped into it and how he just got more and more fed up with how craze the people were over the years.
Scientolgy is a cult that gets you to come in under the pretense that it's an "alternative to psychiatry". Basically they'll show you how successful other scientologists are and how they turned their lives around and they give you life tips on how to better yourself. Usually it's obvious stuff that you can get from any dumbass pep talk.
But the key difference is that they have a long process of purification that requires money. So basically the more money you pay, the more tests you get and the more they let you into the organization. Once you get to a certain level, they make you give other people tests and they make you share all your secrets and shame.
That's why people who get caught into it do NOT want to get out. They spill their secrets and they know that if they get out of the Church, those secrets will be used against them to destroy their careers and their lives.
Scientology also has a cult mentality of the "us vs them" ( kind of like Mormons ). Once you join, they will instruct and HEAVILY encourage you to either convert all your friends and family OR to sever all ties to non-scientologists.
And once you get deeper into the cult, then it gets stupider with the whole aliens and Xenu bullshit. But at the base, it's just a cult meant to prey on people who are weak, depressed and who are looking to better their lives through a Tony Robbins-esque self-motivation process and through creating bonds with other humans.
Once they sink their teeths into you, that's it. You're done. they will ask for all your time and money and will NOT let you leave. They learn techniques to avoid talking to people in actual conversations. You can see it in the second video. They are instructed to always look into the person's eyes, always talk into their face and shift the conversation to problems the OTHER person has instead of discussing anything pertaining to scientology.
So if I said to a scientologist he was a dumbass, he'd confront me by trying to figure out if I'm a rapist and by staring right into my face to try and intimidate me.
And when that doesn't work, they threaten violence and have been known to carry out on it. They will go after you, your family and your career and they are not afraid to sue anyone who badmouths them.
Basically, stay as far away from these assholes as possible.