A primary source documents shows direct, immediate or firsthand knowledge of a subject or event. Primary source documents are written at the time or on the scene where an event occurred.
Secondary sources lack direct knowledge of a topic or event. These include biographies, monographs, and general periodical articles. Secondary sources are written by people who did not witness or experience an event but may have a great deal of knowledge about the topic.
Historians use primary sources to develop secondary sources!