An appendix is a supplementary section at the end of a document that includes supporting information not essential to the main text, enhancing the clarity and credibility of research. It can contain surveys, interviews, correspondence, research tools, non-textual items, and raw statistical data, and should be formatted according to specific citation styles like APA, MLA, or Chicago. Adhering to guidelines ensures the appendix is organized, concise, and effectively aids in comprehension and research validation.