Fair Use is when an exception or a limitation is taken to the copyright law that stands, regarding any material that is copyrighted by an author. Fair Use is the permission that is granted to an educator or scholar, to make limited use of any material in the classroom or for study purposes, that is copyrighted by an author. Examples of fair use would be, a student, photocopying one chapter or fifteen pages out of a copyrighted work that has twenty chapters and one hundred and fifty pages because they need to read it for a class; or an educator, showing a ten minute clip in a class, of a 90 minute video, and following it up with a discussion in class about what students saw.
Useful copyright links and descriptive explanations
Video explaining the basics of copyright and fair use.
[Via Common Sense Education.]