Plagiarism involves taking another person's words or ideas and representing them as your own. Essentially, plagiarism is dishonest and is a form of cheating because you have falsely claimed ownership of another author's work.
Obvious plagiarism includes buying a paper or copying work from a book or the Internet and putting your name on it without crediting the real author. A less apparent example is paraphrasing another author's idea without giving proper attribution--this is theft of the author's intellectual property and is dishonest.
Most schools have policies detailing the problems and consequences of plagiarism. For example, the University of California at Davis explains that plagiarism's effects are far ranging, including tarnishing a school's reputation, damaging the student's academic career and creating serious legal problems.
Whenever thoughts, ideas or words are borrowed from another source, that source must be cited. The Purdue Online Writing Lab offers guidelines on how to know what information must be cited and how to cite it.
Teachers should establish concrete rules and consequences for plagiarism, talk extensively about the problem and offer students examples of the many kinds of plagiarism.