The concept of fair use is somewhat vague, and its application depends on the facts of the particular case. Fair use is a gray area that is not definitively described in either case or statutory law.
Common Myths
Many think "educational purposes" is a blanket defense that can excuse an educator from infringement of copyright law. This is not true. Even educators need to get permission to copy works that they intend to distribute in class.
Availability online does not make it part of the public domain. Internet materials are protected by copyright law.
Out-of-print books, depending on the date of publication (see the public domain chart), may also be protected. Courts have viewed these instances as the copyright holder's only recourse to compensation for creation of the work, and it is particularly important to seek permission in these cases.
The fact that a work will be used for educational purposes will tip the scales toward fair use, but if distributing copies in class eliminates all the class members from the potential market, the scales may tip back.