At UK, plagiarism is a serious offense with consequences ranging from receiving a zero on an assignment to expulsion.
"Plagiarism means taking the words and thoughts of others (their ideas, concepts, images, sentences, and so forth) and using them as if they were your own, without crediting the author or citing the source." (Source: Plagiarism: What Is It, UK Office of Academic Ombud Services)
This paper clearly explains plagiarism, provides examples of both good and bad paraphrasing, and tips on how to avoid plagiarism.
To avoid plagiarizing someone else words or ideas, make sure you:
Video used with permission from Cooperative Library Instruction Project under Creative Commons license cc-by-nc-sa.