Plagiarism happens whenever you represent someone else's ideas as your own. There are many degrees of plagiarism. In general, we think of plagiarism as an intentional act of copying someone else's work word-for-word, and occasionally this does happen. However, more often, plagiarism occurs when you use someone else's word or ideas without properly paraphrasing and/or crediting the source you used. Plagiarism is a serious academic crime, usually resulting in a grade of "F" on the assignment, but it can also result in suspension or expulsion from school.
Citing your sources as directed by the three provided online citation guides is the best way to avoid plagiarism. Careful research methods and proper note taking can also help you avoid plagiarism.