Periodicals - any type of source that is printed regularly with new articles each time.
Popular periodicals are the kind you would buy to read for fun. They may have some value for research, depending on the topic, but do not include references so you can check the facts.
Scholarly/Academic periodicals are written for and by people who work in academics: professors, researchers, undergraduate or graduate students. This type of article is best suited for your research because it is reliable and authoritative. Many of these are peer-reviewed.
Trade publications are written by specialists in an industry for people in that industry. While sources are mentioned, they are rarely formally cited. These are not considered scholarly sources.
Look on this page for more information about how to decide what type of periodical it is!