This article is not intended as criticism. Rather, this article is a matter of context - it exists here, on this admittedly and specifically critial site, only so that the reader will be able to make sense of what follows.
The Source
L. Ron Hubbard, once known for his science fiction, created Scientology almost single-handedly. He is sometimes called "Source" by Scientologists, and alteration of his works in the field, except for the purpose of restoration, is considered anathema to good Scientology; doing so is referred to as "squirreling".
The Books
Hubbard was a very prolific writer even before he began his work on Scientology, and this didn't change. The doctrine of Scientology is contained in dozens of books, manuals, lectures, and the like. There is every indication that it was his intent for the reader to progress through these in a specific order - though why this is the case is a matter of dispute between Scientologists and critics. The basic books include the fairly iconic "Diantetics" and "The Way To Happiness".
The Church
As Hubbard wrote the materials that would become the foundation of Scientology doctrine, he worked to form the overall organisation of Scientology around him. This overall organisation is hierarchal, and progress within it is based strongly on knowledge of and comforty with the materials and on statistical analysis of the effectiveness of the participant (a member that is effective is "upstat", where one that is ineffective is "downstat"). As a hierarchy, the church practices strict codes of behaviour, which are generally termed "ethics". It holds and maintains significant property, and they are many groups tied to the church through their liscensing of "Hubbard Tech".
Basic Teachings
The very basic doctrine of Scientology is that a person is a Thetan (a spirit, broadly, though the definition has more subtext than that) that is currently in possession of and operating a body. Also, that Thetans are by nature endowed with abilities that humans generally lack. Access to these abilities is blocked by unclear thinking, by memory-association, and by denial of the nature of reality. Scientology promises methods to aid the adherent to "clear" these blockages and regain their natural abilities as a Thetan.
Basic Practice
The most practice in Scientology is Auditing, which involves recalling painful or confused memeories in detail and "talking through them" in an attempt to discharge blckages and associations attached. This process is administered by another Scientologist, called an "Auditor", making use of a galvanic meter known as an "E-Meter", which bears a crude electronic resemblance to a lie detector, used to measure the tension of the person being Audited, in order to determine if they have "discharged" harmful difficulties.
...There's quite a bit more.
The practices and beliefs of Scientology are, as might be obvious, far more extensive than have been described thus far. But this is a critical journal, and what has been noted thus far is enough that the criticisms which follow will have at least some context for the reader.
Additional References
1. The official "What is Scientology?" may be found on their home page, HERE.
2. A strongly critical overview of Scientology may be on "Operation Clambake", found HERE.