A periodical refers to a publication that publishes on a regular or on a 'periodic' basis regardless of whether that be every day, every week, every month, every two months, etc. Journals and magazines are types of periodicals.

A journal refers to more scholarly or academic periodicals that often cover a specific subject area (e.g. American Journal of Public Health, Journal of Dentistry for Children, etc.).

A magazine refers to those periodicals that cover general subject areas or popular culture topics and are directed at a mainstream audience (e.g. Rolling Stone, Time, Cosmopolitan, etc.).