One definition of peer review states that it is “a process by which a scholarly work (such as a paper or a research proposal) is checked by a group of experts in the same field to make sure it meets the necessary standards before it is published or accepted”.
How "peer review" works.
After an article has been submitted to a publisher, the editor of the publication sends it to two or three reviewers (in other words ‘peers’ who are experts in the same field) who then provide feedback on the article. This may result in it being sent back to the authors for further revision before the process starts again, or the article is rejected. Once an article meets the editorial standards it is accepted for publication. See the University of Berkeley "Understanding Science" site for a flow chart and explanation of the peer review process.
Why does it matter?
The peer review process means that reviewed articles are considered more reliable as academic sources as they have been read and evaluated by experts in the field prior to publication. This ensures that the research reported in the article has been checked for any bias or errors. The integrity of the reported research means that this can inform other interested readers or researchers of new developments in the field and enhance or update the body of knowledge on a topic.
Types of peer review:
Blind – reviewers are anonymous
Double blind – both reviewer and author are anonymous
Open review – both reviewers and authors are known.
See also publisher site for definitions.
How to tell when something is peer reviewed.
Some Library databases have an additional limit button for peer reviewed articles which may also be listed as academic or scholarly depending on the discipline. If you are not sure whether a journal has a peer review process, then the Library also has a database called Ulrichsweb: global serials directory. Just put in the title of the journal you want to check and look for the refereed symbol (looks like a small graduation gown) in the third column.
Check your understanding of peer review by trying this Library tutorial.
Image: AJC under CC licence 2.0