MLA: is based on the Modern Language Association’s MLA Manual & Guide to Scholarly Publishing (LB 2369 .G53 2003 Reference Desk). MLA Style is commonly used in the Arts and Humanities, which is why it is frequently taught in English classes.
APA: is based on the Publication Manual of the American Psychological Association (BF 76.7 .P83 2001 Reference Desk). APA Style is commonly used in the Social and Applied Sciences, Psychology, and Education.
Chicago: is derived from the The Chicago Manual of Style, published by the University of Chicago. Chicago Style is often used in history research as well as many other disciplines.
Turabian: Kate Turabian is the author of A Manual for Writers of Term Papers, Theses, and Dissertations, the Chicago Style guide commonly used by students. It is based on the Chicago Style.