The Council of Writing Program Administrators gives the following definition: "In an instructional setting, plagiarism occurs when a writer deliberately uses someone else’s language, ideas, or other original (not common-knowledge) material without acknowledging its source" (WPA, 2009).
Notice how quotes were used above and a citation pointing to the original source was given? That's because the words and ideas of this definition were created by someone else. If this were a paper, a full citation would also be included in the bibliography to help readers find the original source. This allows readers to see the context in which this was written, verify accuracy, and do further research. In addition to this, citing sources lets your reader know that you are not claiming to be the inventor of these ideas.
Aside from doing the right thing simply because it's the right thing to do, giving credit to your sources by citing them is a big part of the research process. Citing reliable sources lets the reader know that you're well-informed and you know what you're talking about. It provides a solid foundation on which to build your argument.
Plagiarism, even it it's a minor offense, is very serious. Instructors are required to report any instances of plagiarism and penalties can range from receiving a failing grade on an assignment to expulsion from school, depending on the severity of the offense. To make sure you don't accidentally plagiarise, take a look at the resources below. They'll help clarify the issue. If you still have questions, contact your instructor or a librarian. We're here to help.