Often psychology instructors will ask you to find a "Peer Reviewed" or "Refereed" article. This is because they want you to find articles that validate a psychological principle, one that can be or has been scientifically tested and can be confirmed by other psychologists.
"Peer Review" is a process that journals use to 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 of the scholarship, its relevance to the field, its appropriateness for the journal, etc.
Publications that don't use peer review (Time, Cosmo, Salon) just rely on the judgement of the authors (not usually experts in the field of psychology) or editors of the publication as to whether an article is up to snuff or not. That's why you can't count on them for solid, scientific scholarship.
In addition, your instructor may ask you to find an article of "original research". Original research is the actual experiment that has been performed to test the psychological principle or confirm a hypothesis. Look for such terms as "methodology", "participants", "results" to determine if your article is original research. There should also be data and statistical results in the article.
Original research is not a literature review or a list of other experiments (annotated bibliography) but it will probably have a reference list at the end and a discussion of past experiments in the beginning of the article.
For further explanation of "Peer Review" see the presentation by the Purdue University Online Writing Lab "Peer Review"
Many scholarly or academic journal articles are studies that utilize statistical methods to either disprove or fail to reject the null hypothesis. These videos walk through typical elements of research methodology that you will encounter in your reading.