When you find some useful ideas in your research and want to use them in your paper, whether they are from books, government documents, magazine or journal articles, non-print materials, Internet sources, TV interviews, or other forms of materials, you need to let your readers know the sources of these ideas, i.e., provide proper reference to each source that you have used, because:
You need to cite each source that you used twice.
Main principle - The reader should be able to easily track down the information sources that you cite.
A citation is formatted in accordance with a recognized, appropriate citation style. As preferences for citation style vary, you should ask your professor or instructor which style you should use.
The following are some commonly used citation styles: