Periodicals is a term used for any materials published at regular intervals or periodically (daily, weekly, monthly, quarterly, etc.) There are several types of periodicals (magazines, journals, newspapers,) found in academic library collections.
Popular periodicals are called magazines (and newspapers) and scholarly periodicals are called journals. Trade jourals are usually specific to an industry or group.
Knowing something about the characteristics of each type--popular magazine, scholarly journal , or trade--will help in identifying the appropriate type of periodical for your research.
Scholarly vs. Popular Periodicals
Sources: Vanderbilt University, Peabody Library; Published on Sep 8, 2017, (YouTube).