A dissertation or thesis is a requirement for many graduate-level programs. They are the written form of original research or creative activity created by the student during their master's or doctoral program.
In the United States, the written product is most often called a thesis at the master's level and a dissertation at the doctoral level. However, in other countries--like Australia--both works are called theses regardless of the graduate program.
You can sometimes find new research results and theories in dissertations and theses before finding them in the journal article literature.
CSUEB Library has an entire database for finding dissertations and theses: Digital Dissertations.
Dissertation and Thesis Characteristics:
Differentiating Dissertations and Theses: