A group of scientists completes a study and writes it up in the form of an article. They submit it to a journal for publication.
The journal's editors send the article to several other scientists who work in the same field (i.e., the "peers" of peer review).
Those reviewers provide feedback on the article and tell the editor whether or not they think the study is of high enough quality to be published.
The authors may then revise their article and resubmit it for consideration.
Only articles that meet good scientific standards (e.g., acknowledge and build upon other work in the field, rely on logical reasoning and well-designed studies, back up claims with evidence, etc.) are accepted for publication.
- List taken from Berkeley