In order for an article can be peer-reviewed, it must go through a rigorous "process of critical evaluation by one or more experts on the subject, known as referees, responsible for...evaluating originality, quality of research, clarity of presentation, etc. Changes may be suggested to the author(s) before an article is finally accepted for publication. " (Taken from the Online Dictionary for Library and Information Science).
In other words, peer-review is a process that many scholarly journals undertake to evaluate the quality of the material being submitted. When an article is submitted to be published in a peer-reviewed journal, several impartial reviewers (who are experts in the subject matter) assess for accuracy and the validity of the research methodology and procedure. The article is written by a scholar/expert, is usually long (10+ pages), and has footnotes and an extensive bibliography at the end.
Click here to see a marked-up example of a peer-reviewed journal article. This will show you what to look for in determining if an article is indeed peer-reviewed.