Wikipedia:Classification of sources
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Wikipedia articles are based on three types of sources: primary, secondary and tertiary. These types (primary sources, secondary sources and tertiary sources) arise from the field of Historiography, but their working definition on Wikipedia is precised from the traditional meaning, as suitable to our purposes of summarizing existing scholarship and complying with Wikipedia:Verifiability and Wikipedia:No original research.
What is a primary source?
A primary source is a source that directly reflects an experience—it is documentary in nature. Examples of primary sources are: a photograph, an eyewitness account, an archaeological artifact, a novel or any sort of fictional work. Primary sources are used only when they can enrich the context ascribed to them by secondary sources.
What is a secondary source?
A secondary source is any source that is not a primary source—it is interpretive in nature. Examples of secondary sources are: an analysis of a controversial photograph, a historical study, an archaeological report in a scientific journal, or a book review. Secondary sources are the fundamental building blocks of the encyclopedia.
What is a tertiary source?
A tertiary source is a special case of a secondary source that relies on other secondary sources—it is in the nature of a review. Examples of tertiary sources are: a survey of archaeologists' papers over the last year in a scientific journal, an encyclopedia or other reference work. Wikipedia is itself a tertiary source. Tertiary sources are used only when secondary sources are unavailable, or when it is necessary to discuss the range of scholarly opinion on a subject.
Use-dependent classification
It is not always possible to describe a single source as primary, secondary, or tertiary. The classification depends in part on the use of the source. For example, consider a biography of Alexander the Great written in the time of the Roman Empire. As it informs us on the life of Alexander it is a secondary source, but as it informs us on the world-view of Roman society, it is a primary source.