Citation Style Handbooks

APA Style
Publication Manual of the American Psychological Association, 6th Edition (Find It @ Harvard) Available in print.

Chicago Style
The Chicago Manual of Style, 17th Edition Available in print and online.

MLA Style
MLA Handbook, 8th Edition (Find It @ Harvard) Available in print.

The Bluebook for Legal Citations
The Bluebook: a uniform system of citation, 15th Edition (Find It @ Harvard) Available in print.

Not using any of the above? This University of Maryland Guide provides a comprehensive list of citation styles by discipline.
Once you identify the handbook you need you can locate it at Harvard using HOLLIS+.

Online Guides

Purdue University's Online Writing Lab (OWL)

A great series of guides that help quickly answer citation and writing questions.

Online Resources for APA

APA: Style Blog

APA: Frequently Asked Questions about APA Style

Online Resources for MLA

MLA Style Center

Online Resources for Legal Citations

Harvard Law School Library Guide to Legal Citations

Harvard Style

The Harvard Citation Style, also called the Harvard Referencing System or "Author-date Referencing"

The "Harvard System" is something of a misnomer, as there is no official institutional connection.  It's another name for the author/date citation system, the custom of using author and date in parentheses, e.g. (Robbins 1987) to refer readers to the full bibliographic citations in appended bibliographies.  Some Harvard faculty were among the first practitioners in the late 19th century, and the name stuck, particularly in England and the Commonwealth countries. 

For a full explanation, please see the Wikipedia article for Parenthetical References; History.  The definitive scholarly article on the subject is Chernin C. The "Harvard System": a mystery dispelled.  British Medical Journal 297:1062-1063, October 22, 1988.  

If you're looking for authoritative guidance, there are many excellent sources freely available online, and the Chicago Manual of Style has an excellent chapter on Author-Date Referencing.