The function of secondary sources is to interpret primary sources, and so can be described as at least one step removed from the event or phenomenon under review.
(Definition from University of California Santa Cruz, University Library "Distinguish between Primary and Secondary Sources." Accessed March 30, 2016. http://library.ucsc.edu/help/howto/distinguish-between-primary-and-secondary-sources)
Original material created at the time of the event or by the subject you are studying. This kind of material is the closest you can get to your actual subject, unfiltered by later scholars and critics.
Examples of primary sources include:
original works of research
survey and poll data
newspaper accounts of an event as it happened
A work that analyzes primary sources. They can also act as primary sources, depending on your subject.
Examples of secondary sources include:
A source that indexes or otherwise collects primary or secondary sources. These sources tend to be most useful as jumping off points for your research, leading you to the more in-depth secondary and primary material.
Examples of tertiary sources include: