The APA style is mostly used in Psychology, Education and other social sciences.
The Chicago referencing system is used with all subjects and formats including books, newspapers, and other non-scholarly publications and materials that are not intended for publication.
The Harvard style is generally used across many subject disciplines.
The MLA system is mostly used by researchers in Arts and Humanities. However it is especially used in English Language, Literature, Folklore and Lingustic courses.
The Vancouver system, also known as the "author-number" system, is a way of writing references in academic papers. It is popular in the physical sciences, biological sciences, health and is one of two referencing systems normally used in medicine, the other being the Harvard style.