Primary source: Any information or evidence that is contemporary to the event or topic under investigation. These are sources created at the same time the event took place and usually by an eyewitness of the event in question. Primary sources are generally one of a kind and they are not published documents. Examples of primary sources found in manuscript collections include letters, diaries, photographs, account books, minutes of meetings, scrapbooks, etc.
Written descriptions of an event that are based upon primary sources are considered secondary. These usually include historical monographs and other published works.
The difference between primary and secondary sources is not always distinct. What is a primary source for one topic, may be secondary for another. Always keep in mind when the document was created and by whom. For example, a newspaper article written by a correspondant who witnessed the Battle of Vicksburg in 1863 is considered a primary source for a researcher studying the Civil War. A newspaper article written by an historian in 1995 about the same battle is considered a secondary source. Both sources are newspapers, but the former was written by an eye witness of the battle; thus, it is a primay source.