Secondary sources, in the study of history, are those writings which were not penned contemporaneously with the events in question.
For example, if an author reads a book written by someone who did not witness the events and times described at first hand, but only heard or read about them elsewhere, and then uses the information in that book as a source for writing a new history of the same events, the author is using a secondary source.
If, however, the author uses writings by participants and contemporaries--people who have personal, first-hand knowledge of the events in question--the author is using primary sources to write history.
As a general rule, modern historians prefer to go back to primary sources, if available, as well as seeking new ones. A work on history is not likely to be taken seriously if it only cites secondary sources. This of course does not preclude secondary sources being used as a guide to find and interpret primary sources.
