There are three types of sources:

1) Primary Sources

• Original materials that provide direct evidence or first-hand testimony concerning a topic or event.

• Primary sources can be contemporary sources created at the time when the event occurred (e.g., letters and newspaper articles) or later (such as, memoirs and oral history interviews).

• Primary sources may be published or unpublished.  Unpublished sources are unique materials (e.g., family papers) often referred to as archives and manuscripts.

• What constitutes a primary source varies by discipline. How the researcher uses the source generally determines whether it is a primary source or not.

2) Secondary Sources

• Works that interpret, analyze, and discuss the evidence provided by primary sources (e.g., scholarly books and articles).

• Secondary sources are generally a second-hand account or observation at least one step removed from the event.

• Secondary sources, however, can be considered to be primary sources depending on the context of their use. For example, Ken Burns' documentary of the Civil War is a secondary source for Civil War researchers, but a primary source for those studying documentary filmmaking.
  

3) Tertiary Sources

• Books or articles that synthesize or distill primary and secondary sources, often in a convenient, easy-to-read form (e.g., dictionaries, encyclopedias, indexes, and textbooks).