Periodicals include newspapers, popular/general magazines, and scholarly journals. Each of these is published on a regular or periodic (daily, weekly, monthly) basis, hence the name--periodicals.
Journals are the primary means of communication among scholars within a particular field. Scholarly or academic journals usually present original research, and sometimes (but not always) will be peer-reviewed or refereed. Professional or trade journals provide information to those in the field or industry.
General/popular magazines are usually commercially published; they cover current or popular topics, and may be purchased at newstands/grocery stores. Magazines provide good overviews of a topic.
Journals and magazines are important sources of up-to-date information.
Use this guide to learn the differences between general magazines, trade/professional journals, and scholarly journals.
This guide will also help you to determine if the article you've found is peer-reviewed, or refereed.