What is a citation?
A citation identifies for your readers the original source from which you borrowed an idea, image, or other information that now resides in your own project or research paper, web site, or other work. You must cite the following:
A direct quote taken verbatim from another source
Any source you paraphrase, summarize, or from which you obtained ideas
Ideas or facts that are not common knowledge
A basic citation includes the author(s), title, publication date, page numbers, and other "bibliographic" elements arranged in a particular order.
Why do it?
Give yourself some credit. When you cite, you show evidence of your own research and hard work.
Give credit to others (and avoid plagiarism).When you incorporate ideas and information from other authors--whether through direct quotes or paraphrasing--be fair by acknowledging their work.
Become part of the chain of scholarship. Contribute to your readers' own research process by giving them the opportunity to follow up on your source material.