From a historical point of view, most of the materials in Archives & Special Collections are considered primary sources. A primary source is a document or physical object which was written or created during the time period you are studying. These sources offer a contemporary view of a certain event. Examples of primary sources are: diaries, letters, photographs, official documents, interviews, and works of art. Primary sources are created by individuals who were involved in the event or lived in the time period you are studying.
The Library of Congress provides a very good description of how to cite primary sources using Chicago style and MLA style. However, you should always abide by your style manual. For citation assistance, see the library's citation style guide.