Periodical is a generic term for magazines, newspapers, and journals.
Periodical literature is the chief source of current information.
You might be looking for data on a subject which would be too specific or tow narrow to be included in a whole book. The gap in time between publication of facts in a magazine article and a book can be up to one year or more. Scientific information is being produced so rapidly that books are oftentimes not a logical source for reporting these facts. Indexes help you locate the specific article you want in a periodical by using either a subject heading or a specific name or place. Instead of browsing through a stack of journals to find the information you need, you may find the article by checking these indexes.