I found an excellent descripiton of Dianetics on the Scientology Theology web site.

L. Ron Hubbard's path to the founding of the Scientology religion began with certain discoveries he made in his research into the nature of man. He announced his findings in 1948 as "Dianetics," a word which means "through the soul" or what the spirit is doing to the body.

With Dianetics, Mr. Hubbard discovered a previously unknown and harmful part of the mind which contains recordings of past experiences of loss, pain and unconsciousness in the form of mental image pictures. These incidents of spiritual trauma are recorded along with all other experiences of one's life in sequential order on what Scientologists call the time track.