You should cite the source for any idea that was not originally your own. For example:
1) Cite any direct quotation from another author's work. (Also put the quotation inside quotation marks, or if it's a lengthy quote, indent it.)
2) Cite any idea that originated with someone else that you have paraphrased in your paper. (See Chapter Seven for more information about paraphrasing.)
3) Cite any source that you refer to in the course of your paper, even if you do not quote from it or paraphrase its content.
In each case, remember that the point is both to give credit to the originator of the content or the idea and to show your readers how to find the source for themselves.