Plagiarism is using someone else's work without giving proper credit. Plagiarism is a crime because it is stealing someone else's work, so don't do it! Plagiarism includes copying another student's work, using someone else's ideas or words without giving credit, or otherwise turning in someone else's work as your own.
You can avoid plagiarism by:
Want to know more about plagiarism and how to avoid it? Check out What Is Plagiarism? and When Your Writing Isn't Your Own.