Whenever you consult any other resources (i.e. books, articles, people, etc) to prepare a paper or presentation, you must cite or refer to the source of your information. If you don't cite your sources of information, you are in essence presenting their ideas as your own, which is usually considered cheating and/or plagiarism.
There are many different styles of citation, but the primary methods are MLA, APA, and Chicago style. Click on the links below to find out more about each style of citation, and to see many examples of how sources would be cited in your bibliography or references list.