Plagiarism is theft

Plagiarism is presenting the words or ideas of someone else as your own without proper acknowledgment of the source.
When you work on a research paper you will probably find supporting material for your paper from works written by others. It's okay to borrow the ideas of other people, but you do need to correctly credit them. If you don't credit the author, you are committing plagiarism. The only time you don't need to cite the author is when information is common knowledge.