A primary source for history is any piece of information that was created by someone who witnessed or was part of the events that are being described.  These are also usually authoritative documents concerning the subject under consideration.  This includes original accounts, original works, or original research.  Physical objects can also be primary sources.

The most common primary sources are manuscripts, diaries, journals, and letters.  Primary sources can be rare or unique, published, unpublished or non-print material available for research.  Primary sources are exciting because they are still waiting to be interpreted, to have their context explored, and to be figured out.  Their interpretation is up to the researcher.

The definition of primary sources varies depending on the discipline of the person asked to define them.  Historians will include historical records of any kind, as opposed to published histories.  Literary critics will include poems, plays, and novels, as opposed to the works written about them.

As a general rule, modern historians prefer to rely on some primary sources.  A work on history is not likely to be taken seriously if it only cites secondary sources.  This is because they want you to go through the process of interpretation for yourself -- they want you to create something that is, in some way, original.

In order to understand what primary sources are, you must also know what secondary sources are.  Secondary sources are works that interpret or analyze an event or trend.  They are generally at least one step removed from the event.  For example, a book that evaluates and analyzes the Soviet Union's involvement in the Vietnam War would use a combination of primary sources (perhaps newly released government documents from the Soviet Union citing their involvement) and secondary sources (a plethora of books and articles written on their involvement over the course of the last thirty years) to argue its point.