Copyright and plagiarism are similar concepts but they are different in application. You can infringe on a copyright without plagiarizing and plagiarize without infringing on a copyright. For example if you copied parts of a work in the public domain and submitted it as your assignment without credit, you're not infringing on a copyright, but you are still plagiarizing.
Students need to know:
Copyright issues can be complicated! This guide is meant to inform you of common issues surrounding copyright, but it is not legal advice.