APA (American Psychological Association) style is most commonly used to cite sources within the social sciences. These resources refer to the 6th edition of the Publication Manual of the American Psychological Association:
MLA (Modern Language Association) style is most commonly used to cite sources within the arts and humanities. These resources refer to the 8th edition of the MLA Handbook for Writers of Research Papers:
Chicago style is used in books, magazines, newspapers, and other non-scholarly publications. The Chicago Manual of Style focuses on providing guidelines for publishing in general, and Turabian's A Manual for Writers of Research Papers, Theses, and Dissertations focuses specifically on providing guidelines for student writers.