These databases provide articles that have been reviewed by scholars in the relevant field prior to publication. This is a form of quality control that, while not perfect, provides excellent quality control in scholarly discourse.
Please note: Some of these databases cover popular sources (books, book reviews, opinion pieces, etc.) as well as scholarly journals. When you search, you can limit your results to peer-reviewed articles.
Dissertations and theses are books that are written by doctoral and master's degree candidates in order to obtain their degrees. They are peer-reviewed in the sense that they are judged and approved by a committee of scholars, but many professors do not consider them of the same quality as other scholarly literature.