For some research projects, it is important or you may be required to use primary sources, instead of or in addition to secondary sources. So what's the difference?
A primary source is an original object or document - the raw material or first-hand information. Primary sources include historical and legal documents, eyewitness accounts, results of experiments, statistical data, pieces of creative writing, and art objects. In the natural and social sciences, primary sources are often empirical studies - research where an experiment was done or a direct observation was made. The results of empirical studies are typically found in scholarly articles or papers delivered at conferences, so those articles and papers that present the original results are considered primary sources.
A secondary source is something written about a primary source. Secondary sources include comments on, interpretation of, or discussions about the original material. You can think of secondary sources as second-hand information. If I tell you something, I am the primary source. If you tell someone else what I told you, you are the secondary source. Secondary source materials can be articles in newspapers or popular magazines, book or movie reviews, or articles found in scholarly journals that discuss or evaluate someone else's original research.
(Adapted from from 'Primary and Secondary Sources', Ithaca College Library.).