The Association of Research Libraries provides a brochure for faculty and teaching assistants called Know Your Copy Rights: What You Can Do. (Some information, unfortunately, is now out-of-date.)
The Resources for Teaching Faculty – Frequently Asked Questions webpage contains an excellent, six-step checklist for making a “good faith” judgment on "fair use" of copyrighted material. The FAQ is divided into two sections: one for the traditional (face-to-face) classroom, and the other for the online / course-management-hosted classroom.
The Copyright Office of the United States has put out a video about copyright - "What is Copyright?," that may be helpful to view.
The U.S. Copyright Office's website, copyright.gov, also contains many copyright and fair-use resources.
Fair use allows for the use of copyright-protected works for educational purposes, as well as for commentary, parody, news reporting, and research.