Peer review grows out of the 18th century Republic of Letters and the Royal Society, in which papers were submitted & reviewed by scholars before presentation and publication.
Now peer review is a process by which academic work is evaluated and improved before publication.
An article or book is submitted for review to an editor, who then sends it to 2 or 3 external reviewers, who in turn provide feedback and comments. They return the piece to the editor with notes, who then sends it back to the author. This process may repeat. This process improves the quality of a work, but also means publication takes Years after the work is written.
In Academe, we are especially interested in a specific type of secondary source: Peer Reviewed Publications.
But what's the difference between scholarly, peer reviewed, popular, or refereed?
Scholarly: A source written by an expert, but not subjected to the peer review process.
Example: magazine articles (if written by expert), public presentations, reviews, opinion pieces.
Refereed: Academic work that has some level of vetting, usually by an editor or panel.
Example: Conference papers, journal articles that are approved by an editor but not external reviewers.
NOTE: refereed is often used interchangeably with Peer Reviewed by databases, but it isn't always the same.
Popular: Written for a wide readership. May or may not be written by a subject expert.
Examples: newspaper or magazine, popular press books, websites.
Peer Reviewed: Academic books and articles written by a specialist, reviewed by other experts, and published by an academic press.
Examples: Articles in academic journals, some conference papers, books published by university presses.