It is important to give people credit for their work. When you cite a source you used in your research you are giving credit to the person whose idea you used or refer to. When you properly cite sources, you are demonstrating responsible scholarship.
You document the sources you use in your research in a works cited list or bibliography at the end of your project. For some projects you will include footnotes or in-text citations wherever you use someone else's idea or language.
What's in a citation
Citations include the information necessary for a reader to locate the original source. There are conventions for different disciplines as to the format of the citations. The American Psychological Associaton sets the conventions for APA style, for example.