We all rely heavily on the work of others as we go about our own research and writing. If information of a common, general nature obtained through background reading is incorporated into a paper in our own words, plagiarism is not involved. On the other hand, it is crucial that you be candid 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. There are degrees of plagiarism.
Complete plagiarism involves submitting a paper, essay, review, etc.
The reproduction 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 the writing in a sentence or two as one's own, when it is not. 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.