Check out this video on "Primary, Secondary, Tertiary Resources" from the Beam Library at Oklahoma Christian University.
PRIMARY: materials represent an original work (text, painting, or item) and/or document an event at its occurence (photograph, diary entry, or letter).
SECONDARY: sources are analyses of, commentary on, critiques about, or responses to Primary Sources. Reputable secondary materials have been well-vetted via the peer-reviewed process.
TERTIARY: items are often compilations of excerpts from Secondary Sources and can be used to identify themes or threads pertaning to concepts in a discipline. While subject-specific encyclopedias and dictionaries are ideal for supplemental information about a topic, these types of tertiary sources should only be consulted for initial research and not for citation.