Primary sources are original materials/information in which other research is based. This includes original articles, clinical trials, clinical studies, randomized trials, empirical studies, quantitative studies and case reports.
Secondary sources are analyze, evaluate, interpret, and then summarize this findings/information. This includes systematic reviews and meta-analysis, article and journal reviews, synopses, and textbooks.
When searching in Medline, PubMed, or PsycINFO for primary sources, make sure you are looking at clinical trials, studies, or queries.
If you see the words, "Review Articles" or "EMB Reviews" or "Literature Review" you are looking at secondary sources.