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, it is important 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.
Primary Sources are original materials from the time period involved, and have not been filtered, influenced or analyzed through interpretation. They bring us as close to the original event or thought as possible.
Secondary Sources build on the primary sources with more extensive and in-depth analyses. They summarize, evaluate, and analytically interpret primary material, sometimes by offering a personal perspective. Secondary sources are one step removed from the original source.
Tertiary Sources are distillations and collections of primary and secondary sources. The information is compiled and digested into factual representation, so that it does not obviously reflect points of view, critiques or persuasions. Tertiary sources are typically the last to be published in the information cycle.