There exist several style guides that prescribe the format of written work for professionals and students. The ability to follow these guides is one of the signs of professional quality work, and professors often assign significant grade value to correct formatting, including citations and references.
Citations are the labels, either in the text or in footnotes, that mark a piece of information as coming from somewhere other than the writer's own experience and creativity. These labels give readers just enough information to locate the source in a reference list, where they can see all of the information needed to find the source for themselves. Both the citations and the references have prescribed formats, and a good writer must follow them exactly.
Still have questions?
There are many other styles and variations upon styles that you may be asked to use, to the level of a particular publication's unique or "house" style. You may also be asked to use a "Harvard style," which is an umbrella term for all author-date styles. For clarification or help with unusual requirements, you can ask your professors or an Academic Success Centre instructor.