As you have probably noticed, professors in differing fields have differing expectations for which citation style to use when citing the sources you use for your paper. One reason for the difference is that disciplines have varying interests in emphasizing elements of a source when referring to previous research.
MLA style (Modern Language Association) is most common in the humanities, including your English courses.
APA style (American Psychological Association) is most commonly used in the social sciences. Nursing programs often use this citation style.
AMA (American Medical Association) has become a popular citation style recently for graduate nursing programs.
Chicago style (University of Chicago) is commonly used in historical research, and uses footnotes instead of in-text citations.
Listed below are some excellent guides for helping you to cite your sources correctly.
OWL Purdue- has resources for each of the above styles.
University of Pittsburgh- University Library System
The Chicago Manual of Style Online
Citation Generators- Use at your own risk (in other words, always check for accuracy):