There are at least three reasons why writers cite their sources:
When you are preparing a document,use this checklist to be sure your citations are complete.
Because scholars in different disciplines emphasize different things when they read citations, there are many different styles. The MLA style, used for literary studies, makes sure page numbers are provided in an in-text citaiton because the exactness of a quotation matters; the APA style used in psychology and other social sciences include the year of publication, because when research was conducted is considered particularly significant. The Chicago Style is used by disciplines such as history and religion, which value sources so much it's common to put all the information about a source in a footnote as well as in a bibliography at the end of a paper.
Whatever style you use, citations typically include author, title of the work, and publication informaiton (for books, publisher and year published; for articles, the journal, volume, date, and page numbers; for websites, a URL is needed).