Q. What is a Periodical? What is a serial?

A periodical is a publication that comes out periodically. A synonym is "serial." Magazines, newspapers, and journals are all periodicals. They may come out daily, weekly, monthly, quarterly or annually, but new issues are released during a specific period. Comic books issues also count as a periodical. 

A periodical title will have articles within in it. Sometimes your assignment will call for a specific type of article such as "a newspaper article." 

Periodical issues are indexed by volume and year (these numbers factor into the citation of an article as well) within a database. Databases do not contain every periodical title, but rather a specific set usually. You can see which database holds a periodical title here.