Fair use is an exception created to the copyright laws that sometimes allows people to legally use copyrighted material created by others without obtaining permission "for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" (17 USC §107 et seq.).
The U.S. Copyright Office maintains a Copyright Index of court cases and describes the purpose and limits of fair use. For more instructor resources, see the Library of Congress Copyright and Primary Sources page.