All information taken from a source—all direct quotations, all ideas and opinions, and all precise actual information (for example, statistics and equations)—must be followed by a citation. A citation tells the reader what source provided your information.
Different writing style manuals recommend different ways of citing material. A common form is to put a shortened citation beginning with the name of the source’s writer in parentheses after the borrowed idea or quotation. A longer citation, including all the source’s relevant publication information, then appears in a reference list or works cited page at the end of the paper.
The appropriate style manual will be dictated by your discipline or major. If you are unsure, ask your instructor. If you are writing for publication, the publication's guide for authors will inform you of the appropriate style manual to employ.