The Society of American Archivists (SAA) broadly defines primary sources as "providing compelling, direct evidence of human activity." Users who encounter primary sources gain a unique perspective on the subject they are studying, and an opportunity to learn first hand how primary sources are used for original research.
Primary sources can also be challenging to those who use them. The formats of primary sources may be unique and unfamiliar. They require critical analysis due to their creators’ intents and biases; the variety of contexts in which they have been created, preserved, and made accessible; and the gaps, absences, and silences that may exist in the materials.
By contrast, a secondary source is one that is commenting on or analyzing a primary source, such as scholarly works.
The University of Guelph has an excellent LibGuide defining primary sources with examples and differences between primary and secondary sources as well as a keyword search listing to help find a primary source. Below is a video produced by the University, describing primary sources.
Check out the below links for further information and guidance on how to cite archival materials, including primary sources, images, physical collections, and digital collections.