Give credit to the ideas, words, and works of others
Allow readers to find your sources
Track sources and format bibliographies.
Download citations from library catalogs and databases.
The Purdue OWL (Online Writing Lab) site is an excellent source for questions about how to cite in a particular style. The links below will take you to Purdue OWL's pages for each citation style:
Citation questions? Get help!
The Associated Press Stylebook
(as of February 2020: updated copy on order!)