Citation is the way to credit material or ideas that came from another source. Citation also gives your reader information to locate that source.
Citation style is generally aligned to scholarly discipline, but for coursework, your professor will let you know what style is expected. Some popular styles and their related disciplines are:
APA (American Psychological Association): Commonly used in the social sciences.
CSE (Council of Science Editors): Commonly used in the sciences, including physical and life sciences.
MLA (Modern Language Association): Commonly used in the liberal arts and humanities.
Chicago (Chicago Manual of Style): Has two styles, Notes-Bibliography, used in the humanities, and Author-Date, used in the social sciences.