Sources of information are considered primary, secondary, or tertiary depending on their originality (who did the original work; does it comment on other works) and their proximity (is this a first-hand account, or after the fact). While reviewing literature for a paper or project, you will want to consider whether the literature is a primary, secondary, or tertiary source.
It is not always easy to distinguish between the three types of sources, and they even differ between subjects and disciplines (particularly between the sciences and humanities). By understanding the unique characteristics and features of each, you will be able to identify them and maximize their potential use.
Examples: original research, preprints, letters, correspondence, diaries, court cases, interviews, pictorial works, fiction, poetry, newspaper articles about current events.
Examples: monographs (books), literary critiques, review articles, commentaries, dissertations, thesis, biographies, analyses, public opinion, moral and ethical aspects, history, social and government policy, law and legislation.
Examples: textbooks, encyclopedias, directories, dictionaries, handbooks, guides, classification, chronology, and other fact books.