Answer
A periodical is a publication that is published periodically throughout the year (hence the name!). You may be most familiar with periodicals like magazines, newspapers and journals.
Journals are often issued monthly, but might be published only a few times per year. Magazines are often published weekly or monthly, while newspapers usually go to press daily. Each time a periodical publishes, it contains a unique set of new articles -- no two issues will be exactly alike.
Each issue is numbered, and all of the issues for a year are part of a volume, which is also given a number. That's why, when you cite a specific article from a journal, magazine or newspaper, you must include the volume and issue number. It helps your reader find it more quickly, in case they'd like to read it for themselves.