Plagiarism is when you copy or use someone's ideas and don't give them credit.
When students write academic essays or papers, they are expected to back up their conclusions with evidence, usually from journals and books. It is okay to use ideas that someone else has written but you MUST MUST MUST say where you got the idea. This is called citing. If you plagiarize you may be penalized, or sometimes even kicked out of school.
Citing is done using a style such as APA from the American Psychological Association or MLA from the Modern Languages Association.
Chancellor Paterson Library gives credit to "Illustration and Design: University of Waterloo, Graphics"