Because people claim to have done something that they didn't really do. In colleges people are asked to do research work themselves, so if they just use another person's work and claim that they did...
Because typically they work very hard on creation of the piece or book. Authors are likely trying to make a living with their writing, so if someone else copies the author's work without permission...
Hmm... perhaps when you copy your own words and ideas from something that you own, so it looks like plagiarism but you actually have permission from yourself? :)
Plagiarism (say: play-juh-rih-zem) is when you use someone else's words or ideas and pass them off as your own. It's not allowed in school, college, or beyond, so it's a good idea to learn the proper...
Simply as "Plagiarism is stealing. When you plagiarise something, your taking credit for something you didn't do, and that's lying. So if you get caught plagiarising, not only will you fail that...