A citation is the way you tell your readers that certain material in your work came from another source. It also gives your readers the information necessary to find that source again, including:
Source: Plagiarism.org. (n.d.). What's a citation?. Retrieved from http://www.plagiarism.org/citing-sources/whats-a-citation/
According to (Publication manual of the American Psychological Association, 2010), you have to cite a specific work "whose ideas, theories, or research have directly influenced your work. It may provide key background information, support or dispute your thesis, or offer critical definitions and data. Lastly, Citing of an article implies that the researcher personally read the cited work which directly provides documentation for all facts and figures that are not common knowledge (p. 169)".
(Source: Publication manual of the American Psychological Association (6th ed.). (2010). Washington, DC: American Psychological Association.)
There are many reasons why it is important to cite the resources used in your research paper. Here are some: