The most understood form of plagiarism is the purchase of prepared papers from commercial term paper companies or another individual and the submission of such papers as one's own work.
Another form of plagiarism is a word-for-word copying of someone else's work, in whole or in part, without appropriate acknowledgement, whether that work be a journal or magazine article, a portion of a book, a newspaper piece, another student's paper, or any other composition not your own.
Any such verbatim use of another's work must be acknowledged by (1) appropriate indention or enclosing all such copied portions in quotation marks and by (2) giving the original source in a footnote. As a general rule, you should make very little use of directly quoted matter in your research paper. If you do not know how to footnote properly, ask your instructor for guidance.
A third form of plagiarism is the paraphrasing for the structure and language of another person's work. Changing a few words of another's composition, omitting a few sentences, or changing their order does not constitute original composition and therefore can be given no credit. If such borrowing or paraphrasing is ever necessary, the source must be scrupulously indicated by footnotes. How then, you may ask, can I be original? Am I to learn nothing from others? There are several answers to such questions. Of course you have come to the University to learn, and this means acquiring ideas and exchanging opinions with others. But no idea is ever genuinely learned by copying it down in the phrasing of somebody else. Only when you have the thought through an idea in terms of your own experience can you be said to have learned; and when you have done that, you can develop it on paper as the product of your own mind.