American Psychological Association (APA) style, most commonly used in the social sciences, is a specific formatting style that students and scholars use to write papers and cite sources.
The References list is the final page of a research paper and is comprised of citations for all sources used. It provides the complete details for each source that was cited briefly in-text and should be alphabetized and double-spaced. If the full citation for a source is longer than one line then every line after the first should have a half-inch hanging indent.
Any time you paraphrase or quote from a source you must give an in-text citation. Every in-text citation points a reader to complete details of each source used in the References page (at the end of your paper). In-text citations in APA style require you to give author and year of publication.
Formatting a paper correctly in APA, including margins, page numbers, running head, etc is explained in detail at Purdue University's Online Writing Lab.