MLA style is used mostly by students of the humanities, particularly English, comparative literature, and the performing arts. MLA puts out two publications, each of which cover more or less the same material, but with slightly different emphases. The MLA Style Manual is geared more towards graduate students and faculty. The MLA Handbook, more well known, is usually the choice for undergraduates.
The APA Publication Manual is the style manual of choice for writers, researchers, editors, students, and educators in the social and behavioral sciences, natural sciences, nursing, communications, education, business, engineering, and other fields.