A citation guide is a set of rules for how to cite sources when you're writing an academic paper. Confusion occurs because, in general, different disciplines prefer different citation styles. That's why there isn't just one correct way to cite sources!
For example, the History Department at BU has decided to use Chicago Style. Other departments, such as Psychology, Health Studies, and Education prefer that students use APA Style. The Sociology Faculty usually uses ASA Style, while the English Department leans toward MLA Style.
However, ultimately it's up to you to check with your professors on which style they would like you to use. If you're not sure, always ask!
If you use another author's writing or ideas to create your own paper, you must giving credit to that original author. You do this by providing information on who the author is, which of their works you used, and how someone can find the original publication. If you don't acknowledge the original author by citing the source, you are plagiarising. For details on plagiarism see the Brandon University Policy on Academic Dishonesty and Misconduct.
The Online Writing Lab (OWL) at Purdue University provides information and sample papers for a variety of style guides. They also provide an array of general writing resources and instructional materials.