Define plagiarism. use complete sentences.

The practice of taking someone else's work or ideas and passing them off as one's own.

Plagiarism is when you copy other people's work, without acknowledging it. Including other people work into your own, or even incorporating it without referencing it, means you are owning the work. Here in NZ, if you are caught plagiarising, you receive an automatic Not Achieved. You get no re-submission or resit opportunities. A letter would be sent home, and the markers will assess your work with a lot of precautions.
Although, here in NZ teachers and markers tend to copy and paste sections of your work in google, and if you plagarised, well it shows all there. So be careful. If you intend on using someone else's work, for instance work from websites, books, magazines, speeches and etc, you HAVE TO reference them. 
Plagarising accounts for cheating, and is most definitely wrong. If you were in University, and caught plagarising, there will be BIGGER penalties. 

I hope this helps :)