Reviews summarize and synthesize the current state of the research on a particular topic. Ideally, the writer analyzes all recent literature to provide a coherent narrative of the state of knowledge on that topic. Since reviews do not report new original research, they are part of the Secondary Literature. Review articles will tell you about:
- recent major advances and discoveries
- significant gaps in the research
- current debates
- who's working in a field
- ideas of where research might go next
Review articles typically cover a finite time period (such as the last 5-10 years or since the last major review came out). They are usually longer than research articles (some might be 100 pages or more!) and have extensive bibliographies. Thus they are excellent places to discover important prior articles on a topic. Unlike research articles, reviews are better places to get background on a topic, although some prior familiarity with it is expected.
(Note: Narrative reviews in the physical and natural sciences literature should not be confused with "systematic reviews" found in the biomedical and health science literature. Systematic reviews are completely different in terms of scope and purpose, as well as the methodology behind them.)