"Fair use is the right to use copyrighted material without permission or payment, when the benefit to society is larger than the damage to the copyright holder. It is an essential part of copyright policy in the United States, where the purpose of copyright is to promote the progress through the creation of culture..."

So, the Code of Best Practices in Fair Use for OpenCourseWare (html, pdf) describes fair use, four key guidelines from the 1976 Copyright Act. The guide adds to the set of resources already produced by the Center for Social Media at American University and compiled under the heading Copyright & Fair Use in Teaching and Learning, a noteworthy stop for anyone interested in this subject.