Peer review is a process by which one or more experts in a field read a manuscript, an article, or a book, to determine whether it should be published--usually in academic journals.
Peer review is a type of quality control system for academic research. It confirms that the research process and methodologies used are sound, that the research can be reproduced if necessary, and that sources are properly cited and acknowledged. Depending on the type of peer review process, it can also help reduce bias. Research is published based on its merit--not race, gender, affiliation, etc.
Peer review is normally the gold-standard for academic research, and should normally make up the majority of the secondary sources used in your own academic research.
There are several different types of peer review commonly used in academic publishing.
Blind Peer Review
The author's identity is unknown to the reviewers.
Double-Blind Peer Review
The author and reviewers identities are unknown to each other. This is the common standard for scientific publishing.
Editorial Board Peer Review
Articles are reviewed by an internal board of editors. The author's identity may be unknown or known.
Expert Peer Review
Articles are reviewed by experts whose credentials are known and who are experts within the subject matter of the article under review.