Primary sources are defined, here, by the American Library Association:
"Primary sources are original records created at the time historical events occurred or well after events in the form of memoirs and oral histories…. These sources serve as the raw material to interpret the past, and when they are used along with previous interpretations by historians, they provide the resources necessary for historical research."
--Using Primary Sources on the Web, rev. 2008.
Primary sources are used within the humanities and social sciences as well as in the sciences. In the field of science, a primary source could be an original report of an experiment, observation or finding.
Examples of primary sources:
Secondary sources summarize, analyze, and interpret the evidence derived from primary sources. Secondary sources provide factual context or interpretive framework for analysis. Examples of secondary sources are:
Tertiary resources provide an overview, summary, or index of a topic and may contain both primary and secondary sources. Tertiary sources do not include analysis or critique, but are useful for finding background information on a topic (names, dates, definitions, etc.) and can point to other relevant resources. Examples of tertiary resources are:
Still confused? Check out this video about primary, secondary, and tertiary resources.