APA (American Psychological Association) style originated in 1929, when a group of psychologists, anthropologists, and business managers convened and sought to establish a simple set of procedures, or style rules, that would codify the many components of scientific writing to increase the ease of reading comprehension.
As with other editorial styles, APA style consists of rules or guidelines that a publisher observes to ensure clear and consistent presentation of written material. It concerns uniform use of such elements as headings, tone and length, punctuation and abbreviations, presentation of numbers and statistics, construction of tables and figures, citation of references, and many other elements that are part of a manuscript (Source: Official APA website).
Each in-text citation must be associated with an item in a comprehensive list of references at the end of your paper. Pay attention to your formatting when constructing your reference list; APA is particular about which components to capitalize, italicize, and which punctuation to use in a given situation.