Publication Manual of the American Psychological Association by 
A Manual for Writers of Research Papers, Theses, and Dissertations by It is essential to cite your sources of information when writing an assignment or research project.
There are many different citation styles (APA, Chicago, MLA, etc). Different disciplines use different styles that reflect better the interests of their readers. Generally your professor will let you know which style to use. If your professor has no preference, choose a style yourself and be consistent, use the same style throughout your paper.
Citations give credit for ideas to the originator and allow the reader to track down the original sources of the information. Failure to cite your sources constitutes plagiarism. Plagiarism is a problematic and controversial topic in the sciences, ask your professor when in doubt or check the Academic Honesty Box for additional information.
Check the following video to learn more:
