The Public Domain refers to works that were created in certain time periods that are no longer protected by copyright, and can therefore be shared and used in your class. If a work is in the public domain, you are free to use that work in any way that you choose – digitize it, re-publish it, post it on the web etc. with appropriate attribution.
* For more info, please see Cornell's "Copyright Term and the Public Domain in the United States."
The Public Domain Slider is a tool to help determine the copyright status of a work that is first published in the U.S.
For those works that are still protected by copyright, don’t forget to consider fair use to determine if the work can be used in other more limited ways. The Fair Use Evaluator is a handy tool to help you determine Fair Use.