It's important to cite sources you used in your research for several reasons:
APA stands for the American Psychological Association and is the format designed for use within the field of psychology. However, other disciplines use APA as well, so always use the format your professor chooses.
MLA stands for the Modern Language Association, and its style guidelines have been assisting authors since 1951. MLA style is most commonly used to cite sources within the language arts, cultural studies, and other humanities disciplines. For more information, please consult the official MLA Handbook (9th ed.).
Please note that MLA guidelines do change over time, so it’s important to be aware of the most current information. As always, follow the requirements of your teacher or professor; their requirements take precedence.
Citing Medicine: the NLM style guide for authors, editors, and publishers
The official style manual for the National Library of Medicine (NLM), and suggested when citing documents from the associated databases MedLine and PubMed. The online edition supercedes the print edition as the most up to date.