Primary sources are original materials. They are from the time period involved and have not been filtered through interpretation or evaluation.
Examples:
From: "Primary, Secondary and Tertiary Sources." Univeristy of Maryland, Univeristy Libraries. 1 March 2013. Web. 3 October, 2013.
Secondary sources are accounts written "after the fact" and with the benefit of hindsight (greater understanding of the incident). They are normally interpretions and evaluations of primary sources. Secondary sources are not evidence, since they provide commentary on and discussion of the evidence.
From: "Primary, Secondary and Tertiary Sources." Univeristy of Maryland, Univeristy Libraries. 1 March 2013. Web. 3 October, 2013.