A copy of this book is also available at the Help Desk.
"The rules of APA Style ®, detailed in the Publication Manual of the American Psychological Association, offer sound guidance for writing with simplicity, power, and concision. APA Style has been adapted by many disciplines and is used by writers around the world (apastyle.org)."
In short, APA is one of the many writing styles that are used in the academic world. It is used in many disciplines associated with behavioral and social sciences such as anthropology, economics, and sociology.
There are many differences between APA style and MLA style so be sure to check with your instructor to see what writing style they prefer.