Copyright infringement and plagiarism are two different, but related, issues. Understanding the difference between the two and how they can affect your work can be incredibly useful as you publish your works, particularly if your work is building upon past scholarship and research.
Plagiarism: The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft. This is an ethical offense.
Copyright Infringement: When a copyrighted work is reproduced, distributed, performed, publicly displayed, or made into a derivative work without the legal permission of the copyright owner. This is a legal offense.
It is possible to plagiarize even when you have cleared permission for all the copyrighted works in your thesis, if you forget to cite where those sources came from. Similarly, it is possible to infringe copyright even when you have given careful attribution, if you do not have permission to share the works you are including in your own.
For images, figures, and longer excerpts or copies of a copyrighted work, you will likely need to get permission to reuse the work in your won thesis or dissertation. More information about this can be found on the next pages of this guide.
If you are using a short quote or example from a scholarly work to criticize, review, or provide context for your work (as for a literature review), you do not usually need to get permission from the rights holder. However, whenever you use someone else's work to inform your own, you need to cite where your information came from!
You are likely used to this process by now. You probably learned about citing sources in your introductory English or Writing Communication courses. However, you might not know that you also need to cite sources that you've gotten copyright clearance or permission to include in your thesis/dissertation. When in doubt, cite!