A primary source is a document of direct experience. It's as close as you can get after the fact to having been there yourself.
Secondary sources, like review articles and monographs, rely on primary sources as evidence.
And tertiary sources, like encyclopedias and indexes, summarize and comment on primary and secondary sources. See examples and how they all interrelate in the diagram below.
How do you tell whether the article you're looking at is primary or secondary? Listed below are examples of primary and secondary journal articles to help you gain understanding of what they look like. Compare the source you've found to these examples.