Here is a good definition from the Lloyd Sealy Library at the John Jay College of Criminal Justice
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 Complete: