Here's another question that could fall under the heading of "What is Scientology?" and that is, "How did Scientology Start?" Here's a short history of Scientology:
Basically, L. Ron Hubbard was an engineer who had traveled the world and began studying the mind in 1923. In 1947 he wrote a manuscript laying out some of his discoveries and that manuscript was circulated around by friends, who copied it and passed it along. (This manuscript is now published and is called Dianetics, The Original Thesis.) As copies of the manuscript got passed around more and more broadly, L. Ron Hubbard received more and more letters asking him questions about his discoveries. He decided to write a comprehensive text on the subject to answer all those questions. This is where the book, Dianetics, the Modern Science of Mental Health sprang up from.
When Dianetics was published, people all across the world began to put the techniques that L. Ron Hubbard lays out into practice and after thousands of hours of Dianetics counseling was conducted, evidence had been gathered about the fundamentally spiritual nature of man. L. Ron Hubbard had discovered that man was a spiritual being, inhabiting a body and using a mind. These discoveries made him realize that he had entered the realm of religion.
In 1954, the first church of Scientology was formed in Los Angeles by a group of Scientologists. Within a few years churches had been formed across the country and around the world.
In the years between 1950 and 1986, L. Ron Hubbard finished his research on the spiritual nature of man. These writings are available in most libraries across the US. Further writings are available at Scientology churches.