Written by Elspeth H. Brown, Department of History, Univ. of Toronto.
From UTL, Writing about History: http://advice.writing.utoronto.ca/types-of-writing/history/
A primary source is [an item or] document that was created at the time of the event or subject you've chosen to study, or by people who were observers of or participants in that event or topic....The medium of the primary source can be anything, including written texts, objects, buildings, films, paintings, cartoons, etc. What makes the source a "primary" source is when it was made not what it is.
From: Memorial University Library, Primary Sources Research Guide. guides.library.mun.ca/content.php?pid=401223
works that discuss a subject, but which are written after the time that the event(s) occurred - [by someone other than an eyewitness]
works that contain explanations/judgements/discussions of past events
works that explain or interpret primary sources