A periodical is anything that comes out with regular issues. A daily newspaper, a weekly news magazine, a monthly journal, and an annual book series are all examples of periodicals.
Some periodicals are published in ways that make them more valuable to scholars. Periodicals that are "scholarly", "refereed", or "peer-reviewed" all have articles that are reviewed by experts in the subject. For other types of periodicals, the articles may be reviewed by a professional editor who is not an expert in the subject.