Plagiarism is presenting the words or ideas of someone else as your own without proper acknowledgment of the source. This includes using papers that you have previously submitted without citing them. See the Liberty Way for more information.
If you don't credit the author, you are committing a type of theft called plagiarism.
When you work on a research paper you will probably find supporting material for your paper from works by others. It's okay to use the ideas of other people, but you do need to correctly credit them.