When conducting research for your writing, a “primary source” is the preferred source and the best place to get first-hand information.  For example, a person who can provide or write about a first-hand experience of an event is a primary source of information. Other kinds of primary sources are maps, photographs, drawings, videotapes, diaries, letters, manuscripts and other similar items.

See Yale University’s web site for a good understanding of the differences between primary and secondary sources: http://www.library.yale.edu/instruction/primsource.html

This is also a permanent link under the English Resources page.