The citation styles you use in your academic work will vary, most often dependent on the discipline of the individual course. When in doubt about what you should use, ask your instructor.
A style guide is a set of standards for the writing and design of documents. There are many different kinds of style guides, but three of the most used are the American Psychological Association Publication Manual (APA), the MLA Handbook for Writers of Researcher Papers (MLA), and the Chicago Manual of Style (Chicago).