What's the difference?
"Use APA format" or "Follow MLA guidelines."
What does that mean?
The Modern Language Association (MLA), American Psychological Association (APA), and The University of Chicago Press issue guides to help students and researchers organize their papers and give credit to their sources. MLA style is often used for English papers, APA style is used more often for social science papers, and Turabian(Chicago) is used mostly for history papers.
Always use the style your instructor asks for.
Always check your assignment guidelines/syllabus for style requirements for a course. Your instructor may prefer a different citation style from what is standard for the discipline, or they may have slightly different requirements for their papers. Ex.: They want you to use APA style but with Arial font.