Primary Sources are materials that contain direct evidence, first-hand testimony, or an eyewitness account of a topic or event under investigation. They can be published or unpublished items in any format (the original or a surrogate format such as a photocopy, a digital copy, a printed edition, or a microfilm edition), from handwritten letters, to objects, to the built environment.
Secondary Sources are works that analyze and interpret other sources. They use primary sources to solve research problems.
Primary vs. Secondary - The way you engage with a source determines whether it is a primary or secondary source for your project. Book reviews, for example, are typically considered secondary sources. If the subject of your research is book reviews themselves, however, they would be primary sources for your project.
Unpublished archival materials are just one type of primary source. In The Information-Literate Historian, Jenny Presnell describes nine categories of primary sources:
Source: The Information Literate Historian, 93-95.
Secondary and reference sources will not only help you put your topic in context, they will often lead you directly to archival collections.
Check the footnotes, bibliographies, and acknowledgments in books, dissertations, and other published sources for clues to other sources that might be relevant to your topic. Through this process, you may learn about cataloged and uncataloged materials held in libraries and/or private collections.