A dissertation is the final large research paper, based on original research, for many disciplines to be able to complete a PhD degree. The thesis is the same idea but for a masters degree.
They are often considered scholarly sources since they are closely supervised by a committee, are directed at an academic audience, are extensively researched, follow research methodology, and are cited in other scholarly work. Often the research is newer or answering questions that are more recent, and can help push scholarship in new directions. ( Borrowed from Kate McManus, Magrath Library, University of Minnesota)