Primary sources refer to information collected firsthand from such sources as historical documents, literary texts, artistic works, experiments, surveys, and interviews. Thus, articles where the author is describing their own experiments would be considered a primary source. In science and the social sciences, research articles are considered primary sources. In history, a much wider range of sources would also be included everything from original research and original works of fiction to items created during the period that is being studied. These can include personal papers including diaries, journals, and correspondence; legal notices and records for births, deaths, divorces, marriages, and military; photographs, and jewelry.
Secondary sources refer to another person's second-hand account of something such as in a literature review. Thus, an experiment that is described by someone other than the researcher(s) would be considered a secondary source. Accounts found in newspapers, radio, and television about research findings would be considered secondary sources.
Tertiary sources provide generalized overviews of a topic. The author usually does not go back to the primary sources and instead gathers information based on secondary sources.
APA has specific rules for citing a primary source that is cited using a secondary source.
It is always best to read the original or primary sources, but sometimes this is difficult if the original work was published in another language or was published in a book that is difficult to obtain. In such a case, you would need to cite the original or primary source in the text of the paper, but you would provide a reference in the reference list for the secondary source.
a. Within the Text
According to Freud (as cited in Skinner, 1974), the characteristics...
b. In the Reference List
Skinner, B. F. (1974). About behavioralism. New York, NY: Knopf.
See also the APA Style website's FAQ on how to format a secondary source.