Primary resources are the basic evidence for any good research project. These sources can come in many formats: documents from archives, interviews or eyewitness accounts, letters, diaries, newspaper articles, video footage, etc.

One example: The book Pablo Picasso, Gertrude Stein: Correspondence contains primary resources in the form of written correspondence between Pablo Picasso and Gertrude Stein. These are reproductions of the originals.

Secondary resources

Secondary resources are works that interpret, analyze, and often synthesize primary source materials to give a broader understanding of a certain topic. These may come in the form of 

scholarly books or journals. They often cite primary source materials, but are a step removed from those events. 

One example: The book Pablo Picasso, 1881-1973 is an example of a secondary resource: a scholar presenting criticism and interpretation of the artwork of Pablo Picasso.