According to the Indiana University-Bloomington Library:
A primary source provides direct or firsthand evidence about an event, object, person or work of art. Characteristically, primary sources are contemporary to the events and people described and show minimal or no mediation between the document/artifact and its creator. As to the format, primary source materials can be written and non-written, the latter including sound, picture, and artifact.
A secondary source, in contrast, lacks the immediacy of a primary record. As materials produced sometime after an event happened, they contain information that has been interpreted, commented, analyzed or processed in such a way that it no longer conveys the freshness of the original. History textbooks, dictionaries, encyclopedias, interpretive journal articles, and book reviews are all examples of secondary sources. Secondary sources are often based on primary sources.
This link compares primary vs. secondary sources in various disciplines: