Copyright infringement and plagiarism can be related concepts but are distinctly different situations.

Plagiarism is when someone passes off the work of another person as one’s own or without acknowledgement of the original source.  Plagiarism is avoidable by making sure you always give credit to the original source when using the ideas or works of someone else in your own work. The LibGuide Academic Integrity: Avoiding Plagiarism and Understanding Research Ethics provides an in depth explanation of what plagiarism is and how to avoid it.  Plagiarism is a moral and ethical matter.

Copyright, on the other hand, does not protect ideas. It only protects the expression of those ideas.  A person potentially infringes on copyright when they; copy, distribute, display, perform, or create a derivative work, without the permission of the copyright holder. Copyright infringement is a legal matter.  

As illustrated below it is possible to be both a plagiarist and a copyright infringer or only a plagiarist or a copyright infringer.

Scenario 1 (plagiarism without copyright violation):

If you copy The Waste Land by T.S. Elliot and put your own name on it and turn it in for your poetry composition course, you would be guilty of plagiarism. Since The Waste Land is in the public domain, it is no longer protected by copyright. You are free to make copies, to distribute them, and to preform and display the poem. You can even adapt the poem into a new work. However, by passing the work off as your own, you have plagiarized T.S. Elliot.  

Scenario 2 (copyright violation but not plagiarism):

You are an artist and you create a sculpture that reproduces a famous photograph. You intend to sell your sculpture for thousands of dollars. You give credit to the photographer but have not requested their permission or paid any royalties for reuse. Since you have credited the original source you are not committing plagiarism. However, since you are copying and creating a derivative work without the copyright owner’s permission, you are infringing on their copyright.    

Scenario 3 (both copyright violation and plagiarism):

You copy the lyrics of a song, say for example “The Sign” by Ace of Base, to your Web site and claim those lyrics as your own work. Because "The Sign" is still in copyright, you have violated the copyright on the work and you have also plagiarized the songwriter by claiming the material as your own.