Copyright is about your legal right to use someone else's work. Copyright law states that you cannot publish (or even copy) anything unless you either own the copyright or obtain permission from the copyright holder. When using anything you didn't create yourself, you must assume that the material is copyrighted unless you know otherwise, even if the material lacks the traditional copyright symbol of a single C in a circle.
Plagiarism is about giving proper credit to the work of others. You are required to properly cite anything in any of your assignments that you did not create regardless of any copyright concerns. This is true no matter what format your assignment is (dissertation, website, or brochure) or what material you are using (research article, song, or photograph). The format of your works cited page can be worked out with your professor.
For information on Fair Use please read "General Guidance on Copyright, Plagiarism and Creative Commons" linked below