Peer-reviewed journals publish articles that have been reviewed by experts in the field. 
These highly respected experts (peers, referees, reviewers) read the submitted article and decide if it is acceptable for publication. If the article is acceptable, peer-reviewers may suggest changes or corrections.
Peer-reviewed articles are also known as refereed articles; generally these are found in scholarly journals.
Not all scholarly journal articles are peer-reviewed, however. Additionally, one may occasionally find peer-reviewed articles in professional journals.
At the beginning of a peer-reviewed journal--generally on the inside front cover--you'll see a listing of many editors with their college or university affiliation listed after their names.
These are the peers or referrees who review the articles.
Most (but not all) databases will have a peer-reviewed limiter.
You'll find the peer-reviewed limit box (see image to right) under the Advanced Search option.
Peer-Reviewed limit not available? Check Ulrich's Periodicals Directory to determine if the journal is peer-reviewed.