A primary source is an artifact, a document, a recording, or other source of information that was created at the time under study. It serves as an original source of information about the topic (the first version). A primary source can be a person with direct knowledge of a situation.
So when should someone use primary sources? To read eyewitness accounts or view photographs or video of an event instead of relying on a summarized explanation; to interpret data yourself instead of relying on another's interpretation; to reflect on and analyze works of literature or art instead of relying on another's opinion; to verify claims made in secondary sources.
Some examples of primary sources include:
Search the National Archives for collections of documents that are important to American History.
The Foreign Relations of the United States series presents the official documentary historical record of major U.S. foreign policy decisions and significant diplomatic activity.