Plagiarism is the academic and literary equivalent of robbery, taking someone else's property.
If you copy somebody's test answers, take an essay from a magazine and pass it off as your own, list a well-phrased sentence or two and include them without crediting the author or using quotation marks, or even pass off somebody's good ideas as examples of your own genius, you are guilty of intellectual thievery. If you are caught you should expect punishment or contempt or both. If you suspect a student or group of students of plagiarism, you should consult the Faculty Handbook, Appendix F, Section III, for the appropriate steps to take. In every case, your first step is to gather the evidence documenting the plagiarism and consult with the Director of the Expository Writing Program before confronting the student. The student has the right to appeal the Dean's decision to the Undergraduate Grievance Committee of the Faculty Senate.