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
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.)
Where are reviews published?
Reviews can be found in different kinds of publications.
JOURNALS: Many research journals also publish regular or occasional reviews. Some journals publish only reviews; examples include Chemical Reviews, Accounts of Chemical Research, and Chemical Society Reviews.
BOOKS: Most scientific books (monographs) are essentially long reviews or edited collections of review chapters.
ENCYCLOPEDIAS: Encyclopedia articles are background reviews intended for a more general audience. Examples are the Kirk Othmer Encyclopedia of Chemical Technology and the Encyclopedia of Analytical Chemistry.
REVIEW SERIALS: These publications, which occupy a middle ground between journals and monographs, often have titles like Advances in ..., Progress in ..., or Annual Review of .... Chapters in these serials can be lengthy and thorough summaries of past and recent research. One of the best known examples in chemistry is Organic Reactions, an irregular series that publishes voluminous and thorough reviews of synthesis techniques.