Original research - Written by the researchers who performed the actual, on-the-ground study, these articles include the methods, results and discussion related to a single research project. Original research articles are considered primary sources and are the major form of communication across most of the sciences.
Case study - A detailed analysis of an individual or group as a model for a particular medical phenomena. Impact limited due to sample size, but can serve as an entry point to more detailed research.
Review - An article that summarizes and synthesizes numerous original research articles. These articles are heavily used by researchers seeking to keep up with the literature around a topic. Review articles can be used to identify original articles/authors for further reading. Review articles are generally written by a researcher, but may include information about studies by numerous other authors. Review articles are considered secondary sources.
Systematic review - A systematic method that uses statistics to analyze the results from numerous studies. Systematic reviews are widely used in the health sciences, often to find a generalized conclusion from multiple evidence-based studies. Example: "85% of the studies showed a positive correlation with..."
Meta-analysis - A systematic method that uses statistics to analyze the data from numerous studies. The researcher will combine the actual data from studies with similar data types (i.e. males between 25-30 years old with vitamin D deficiency) and analyze them as a single expanded data set. PubMed/Medline treats meta-analysis as a subset of systematic reviews.
Systematic reviews and meta-analysis articles are a sort of hybrid between primary and secondary sources because they provide new results based on previously published data, and the authors are basing their conclusions on data collected by others.