APA is an abbreviation for American Psychological Association. The APA has established formatting for use in written work within the social sciences. The APA formatting style includes citations within the written work and to the references and endnotes/footnotes.
YourDictionary includes many more tips for formatting in the "APA style" in articles such as APA Outline Format Examples and Writing Tips for APA-Style Research Papers. If these formatting examples and tips do not apply to your particular needs, you can find highly-detailed information about the APA style in the Publication Manual of the American Psychological Association.