The copyright clause of the Constitution gives Congress the power to grant authors exclusive rights to their writings in order to "promote the progress of science and the useful arts" (Article 1, Section 8, Clause 8). The primary purpose of copyrights is to foster the dissemination of works for public welfare. By this method authors are given a "head-start" as a monetary incentive to creative production.
Offsite
Office of General Counsel, University of Texas
http://www.utsystem.edu/ogc/intellectualproperty/offsite.htm
This site has excellent background information on fair use of various
media, as well as scholarly communication, licensing, distance learning,
and general information.
University
of Georgia Regents Guide to Understanding Copyright and Educational Fair
Use
Excellent guide to copyright and fair use with many examples, question-and-answer
sections, and scenarios by media type.