To quote Wikipedia, a citation or bibliographic citation is a reference to a book, article, web page, or other published item, with sufficient detail to identify the item uniquely. Most essays you will write will require citations and a bibliography or work cited page. The rules for creating citations differ depending on the subject of the paper and the rules are found in the appropriate style guide. This guide will cover the main styles of citation.
When you include other people's writing in your paper, you must cite that work. If you don't cite the work, then you have committed an act of plagiarism, which is a serious academic offense. Penalties for plagiarism can include a failing grade for the paper or a failing grade for the course, so don't do it. Cite your sources!
Whenever you use quotes - Cite that source!
Whenever you paraphrase - Cite that source!
Whenever you use an idea that someone else has already expressed - Cite that source!
Whenever you make specific reference to the work of another - Cite that source!
Whenever someone else's work has been critical in developing your own ideas - Cite that source!