The APA is the citation style for Social Science research papers. Scientific subjects covered by the APA include: Pyschology, Sociology, Anthropology, and Statistics.
The MLA Citation (Modern Language Association) style is primarily used for Speech and the Humanities. Humanities topics include: English Literature, World Literature, History, Foreign Languages, Film Studies, and Drama.
Chicago (Turabian) Citation
The Chicago Style, innovated by Kate Turabian, is used by select professors in the Humanities. A rigorous formatting style, it permits for greater variety of citation, provided you explicitly follow the predetermined guidelines.