Citing a source means acknowledging where you got a specific quote, fact, graphic, data, or idea from. A citation is all the information needed to describe a source, such as (but not limited to) the author, title, publisher, page numbers, and publication date. Information in citations follows a specific format or style. There are many different citation styles. Which one you use can depend on what subject or discipline you are writing for or on the preference of you professor.
The following links will take you to sites authored by vendors, organizations, and other academic libraries. These links provide in-depth documentation to the most common style formats.
Chicago - often used in Humanities (English, Arts, History)
Turabian - often used History and other Humanities disciplines
ACS (American Chemical Association); Chemistry
CSE (Council of Science Editors)
AMA (American Marketing Association)
Masters in Bioethics (UGC/Mt Sinai) Project Reference Style Sheet
The library has many of the complete style manuals available in print and can assist you in using them.