What exactly is Scientology?

I have looked it up and have gotten mixed answers. I asked my mom and she gave me a Dianetics book by John L Hubbard (the supposed founder of Scientology), and i'm not sure how that ties into the Scientology I've heard about. Its very difficult to read/focus on. People make fun of this "religion" all the time and I'm always hearing about alien overlords and all kinds of nonsense. Why are people so compelled to follow it? Why is it considered a cult? Can someone explain to me *exactly* what Scientology is?