Plagiarism is literary theft.
A plagiarist often merely changes a few words or rearranges the words in the source. As you take notes and as you write your paper, be especially careful to avoid plagiarism. Unless you are quoting directly, avoid entirely the sentence patterns of the source. If you include a quote, you must use the exact words of the author or it is a misquote. Use quotations only when it is absolutely essential for the reader to know exactly what that particular person said word for word.