Primary sources, for historical purposes, are most often defined as eye-witness accounts of events/historical periods. Those accounts written or created at the time, not with 20/20 hindsight.
These sources reflect the point of view of a participant or observer at a particular point of time. There are a wide range of source materials available for historical research. For example:
Compilations of primary sources such as a book containing many short excerpts from primary sources may not always be as useful as the original sources themselves.
Source materials may have a bias or purpose which you should be aware of. For example, an article from a northern paper during the civil war will read much differently than an article from a southern paper. When reading primary sources, think about why the piece was written, when it was written, and what it tells us about the writer and the time period.