Peer review is a process journals use to help ensure the articles they publish represent the best scholarship currently available. When an article is submitted to a peer reviewed journal, the editors send it out to other scholars in the same field (the author's peers) to get their opinion on the quality and relevance of the work, its appropriateness for that journal, etc. The reviewers' comments are anonymously returned to the author, who makes necessary revisions, after which the paper is accepted and published. This process can take months to complete.
Publications that don't use peer review rely on the judgment of the editors whether an article is worthy. Trade magazines, popular periodicals, and most secondary/tertiary literature (books, etc.) are typically NOT peer reviewed. Some examples: