Primary source
             In scholarship, a document or record containing firsthand information or original data on a topic. Primary sources include original manuscripts, periodical articles reporting original research or thought, diaries, memoirs, letters, journals, photographs, interviews, government documents, public records, eyewitness accounts, newspaper clippings, etc.

Secondary source
             Any published or unpublished work that is one step removed from the original source, usually describing, summarizing, analyzing, or evaluating primary source materials. A review, critical analysis, second-person account, or biographical or historical study are examples of secondary sources. Also refers to material other than primary sources used in the preparation of a written work.

Tertiary source
             A written work, such as a chapter in a textbook or entry in a reference book, based entirely on secondary sources, rather than on original research involving primary documents. Whether a source is secondary or tertiary can be determined by examining the bibliography (if one is provided). Another clue is that secondary sources are almost always written by experts, but tertiary sources may be written by staff writers who have an interest in the topic but are not scholars on the subject.

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