List courtesy of NYU Libraries
A primary source is the "raw" information about which researchers write directly. These sources were produced at the time of an event, or later as a memoir. Examples include correspondence, photos, and newspaper articles.
A secondary source is a source about an event, person, or place, usually based on the analysis or interpretation of primary source materials. Examples include academic articles and books. Keep in mind that a secondary source could be a primary source depending on your research focus. For example, a book about U.S. history written in the 1950s is a secondary source, unless you are studying how history was studied in the 1950s, in which case the book becomes a primary source.
A tertiary source identifies or synthesizes the information of secondary and primary sources. Examples include encyclopedias, dictionaries, and bibliographies.
Chart from Loyola Marymount University's Primary Sources Research Guide.