When writing research papers, documenting your sources is an important step - crediting others whose work you have based your research upon is an integral part of scholarship.
Citing your sources in the proper or accepted format helps the reader to identify these sources. A list of references can be likened to a genealogy which tells others the history and background of your work.
There are many different citation styles - when writing for a journal or other publication, the information is usually provided in an "information for authors" area. For school, your instructor will usually tell you which citation style to use - if not, you should ask.
Citation Styles video (4 minutes)
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Summarizing? Paraphrasing? Quoting?
You can, and should, talk to your instructor, but this video can give you an overview. The first 1:30 is brief description of each, followed by more in-depth explanations.
Incorporating Sources video (4 minutes)