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, interpretations 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.
Although scientific and other peer reviewed journals are excellent sources for primary research, not every article in those journals will be a research article. Content may also include book reviews, editorials, and review articles. Since review articles include citations and are often quite lengthy, on first glance, they can be difficult to differentiate from original research articles. Since the authors of review articles are discussing, analyzing, and evaluating others' research, not reporting on their own research, review articles are not primary sources. They can be of great value, however, for identifying potentially good primary sources.
Primary research articles can be identified by a commonly used format. Look for sections titled Methods (sometimes with variations, such as Materials and Methods), Results (usually followed with charts and statistical tables), and Discussion. Since a review of the literature is part of the research process, the article will also include bibliographic citations and a Works Cited section at the end. An Abstract at the beginning will summarize the research findings and give you a good sense of the kind of article that is being presented, so this is an excellent tool to use to determine if the item is a review article or a research article. If there is no abstract at all, that in itself may be a sign that it is not a primary resource. Short research articles, such as those found in Science and similar scientific publications that mix news, editorials, and forums with research reports, however, may not include any of those elements. In those cases look at the words the authors use, phrases such as "we tested" and "in our study, we measured" will tell you that the article is reporting on original research.