Here is a good definition from the Lloyd Sealy Library at the John Jay College of Criminal Justice
(http://guides.lib.jjay.cuny.edu/evaluating sources):
In academic publishing, the goal of peer review is to assess the quality of articles submitted for publication in a scholarly journal. Before an article is deemed appropriate to be published in a peer-reviewed journal, it must undergo the following process:
· Because a peer-reviewed journal will not publish articles that fail to meet the standards established for a given discipline, peer-reviewed articles that are accepted for publication exemplify the best research practices in a field.
In most databases, you can limit the results to scholarly/peer reviewed articles either from the start of the search or after retrieving the results.
Example from Academic Search Ultimate: