A citation is your way of telling readers how to find the information, data, opinions, and quotes upon which you based your paper.
The following video does a great job of briefly explaining what citation is and why it's important.
MLA stands for Modern Language Association. MLA style was created by scholars to standardize writing in the humanities and liberal arts. Some fields that use MLA format include: English, Literature, and Art.
APA stands for American Psychological Association. APA style was created by social scientists to standardize scientific writing. Some fields that use APA format include: Allied Health, Business, Criminal Justice, Education, Political Science, and Social Work.
Citation: A reference to another author's work, whether directly quoted, paraphrased, or mentioned in your paper. You will need to include a brief in-text citation for the source you've used, and include the full citation information needed to locate that source in your Works Cited or References list. A correctly formatted citation makes it easy for your instructor or readers to find any article, book, movie, or other source that you've quoted or referenced in your paper.