Essentially, peer review is an academic term for quality control. Each article published in a peer-reviewed journal has been closely examined by a panel of reviewers who are experts on the article's topic (that is, the author’s professional peers…hence the term "peer review"). The reviewers look for proper use of research methods, significance of the paper’s contribution to existing literature, and integration of previous authors’ work on the topic (including citations). Papers published in these journals are expert-approved…and the most authoritative sources of information for college-level research papers.
Articles from popular publications, on the other hand (like magazines, newspapers or many sites on the Internet), are written for general audiences. While they are edited for spelling, grammar, and style, they have not been reviewed by experts on the topic in question. Popular sources can be interesting to read, but they are not sufficient for college-level research.