Published literature can be either peer-reviewed or non-peer-reviewed. Official research reports are almost always peer reviewed while a journal's other content is usually not. In the health sciences, official research can be primary, secondary, or even tertiary. It can be an original experiment or investigation (primary), an analysis or evaluation of primary research (secondary), or findings that compile secondary research (tertiary). If you are doing research yourself, then primary or secondary sources can reveal more in-depth information.
Primary research is information presented in its original form without interpretation by other researchers. While it may acknowledge previous studies or sources, it always presents original thinking, reports on discoveries, or new information about a topic.
Health sciences research that is primary includes both experimental trials and observational studies where subjects may be tested for outcomes or investigated to gain relevant insight. Randomized Controlled Trials are the most prominent experimental design because randomized subjects offer the most compelling evidence for the effectiveness of an intervention. See the below graphic and below powerpoint for further information on primary research studies.