Primary sources are distinct from a secondary sources, which are authored by someone who may have had access to primary documents, but did not experience the event directly. Secondary sources include textbooks and other academic studies.
The value of primary sources lies in their immediacy. A primary source provides direct and unfiltered information on the topic of study, allowing the reader to experience the event as personally as possible.
On the other hand, this does not mean that primary sources are necessarily more accurate or "true" than secondary sources. Primary documents tend to be influenced by "source bias," that is, a strong tendency on the part of the author to present the material in a particular way. Secondary sources tend to be much better at balancing information, especially scholarly works that undergo peer review.
Examples of primary sources include memoirs, journals, private correspondence, photographs, statistical studies and the data of scientific experiments. The precise definition of primary vs. secondary sources varies slightly from discipline to discipline.
Primary sources require special treatment to guard against forgeries and data fabrication.