I am a Scientologist of some 40 odd years having completed the bridge and currently run and administer an independent Scientologist group I can probably contribute some useful input. I will start with an extract from my forthcoming book on the subject. "The word Dianetics comes from the Greek DIA through, NOUS soul and deals with a system of mental image pictures in relation to psychic (spiritual) trauma. The mental image pictures are believed on the basis of personal revelation to be comprised of mental activity created and formed by the spirit, and not by the body or brain. It addresses the body as distinct to the spirit or individual (Called the thetan in Scientology – from the Greek word Theta). It addresses the causes of illnesses, unwanted sensations, misemotions (inappropriate emotions for the situation) somatics, pain and so forth.

“Dianetics is not psychiatry. It is not psychoanalysis. It is not psychology. It is not personal relations. It is not hypnotism. It is a science of mind.”
Scientology Technical Dictionary

Hubbard described Dianetics as "an organized science of thought built on definite axioms: statements of natural laws on the order of those of the physical sciences". This and other Dianetic axioms can be found in many Hubbard books such as Scientology 0-8: The Book of Basics and Advanced Procedures and Axioms. The very first book on Dianetics, the one that started it all was Dianetics: The Modern Science of Mental Health. First published in 1950 it went to the top of the best seller list. Here Hubbard recorded his discoveries and application of Dianetics.
Hubbard said, Dianetics would work every time if applied properly and "will invariably cure all psychosomatic ills and human aberrations." In April 1950, before the public release of the best seller Dianetics: The Modern Science of Mental Health, he wrote: "To date, over two hundred patients have been treated; of those two hundred, two hundred cures have been obtained."
In Dianetics, the human mind was found to be a collection of "mental image pictures," which contain the recorded memory of a past moment, including all sensory perceptions and feelings involved. One type of mental image picture, created during a period of unconsciousness, involves the memory of a painful experience. Hubbard called this memory an engram, and defined it as "a complete recording of a moment of unconsciousness containing physical pain or painful emotion and all perceptions."
The application of Dianetics occurs when an auditor (listener) applies specific questions to the preclear (recipient) in order to assist that person find out areas of trauma in their life that causes discomfort.
Dianetics is different to scientology in that it deals exclusively with the mind whereas scientology deals primarily with the being.
The best way of understanding Dianetics is by reading the two definitive works on the subject, Dianetics: The Modern Science of Mental Health and Dianetics Today."