APA style is a set of rules for formatting your papers. These rules are outlined in the Publication Manual of the American Psychological Association. There are two main componets of APA style: in-text citations and a References list at the end of the paper. These components allow the reader to retrace your research.
Every time you use a fact, quote, opinion, etc. from a source, you need to cite the source within your sentence. The References page contains an alphabetical list by author of all the sources you used in your paper.
The in-text citations act as signs for the reader. When the reader sees an in-text citation, he/she can turn to your References page and find all the information needed to locate the original source. All sources in the References list must be cited in-text and all in-text citation must have a corresponding References list entry (with the exception of personal communication, which is only cited in-text).
Other components on APA style are:
The Davenport University Brief Guide to APA contains detailed information on how to to set up your paper. The Student Guide To APA 6th Edition (powerpoint presentation) also contains detailed information.