Obviously, we all rely heavily on the work of others as we do our own research and writing. If information of a common, general nature obtained through background reading is incorporated into a paper in your own words, plagiarism is not involved. It is, however, crucial that you be honest when presenting specific facts, interpretations, or analyses of data which are not your own. If parts of another writer's material are used directly, either paraphrased or directly quoted, without acknowledgment, you are guilty of plagiarism. I am of the school of thought that believes there are degrees of plagiarism.
Complete plagiarism involves submitting a paper that is entirely the work of another person.
Your use of a lengthy passage without acknowledgment, even though a few words may be changed in each sentence, also constitutes substantial plagiarism. Minor plagiarism exists when one presents another's writing in a sentence or two of a paper as one's own. I do not view plagiarism lightly. It is a serious academic offense, and will be treated as such. The penalty for minor plagiarism is normally a reduction in the grade assigned for the assignment in question.