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 = 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.