A citation gives credit to the original author(s) of a work. Citations also allow people who are reading your work to be able to find the original sources of information.
Basic citations for a book, for example, include the name(s) of author(s) or editor(s), the title of the book, the publisher's name, the place of publication, and the most recent copyright year.
There are a number of styles that can be used to construct citations. Each style specifies the information to be included in the citation, the order of the information,the format, and the punctuation.
Your instructor may require a particular style. If there is not an assigned style, then choose a style and be consistent with that style throughout your work.
When reading a citation, it can be difficult to tell if the citation is for a journal article, a book, or a book chapter. Here's an example of an article citation: