What is copyright?
Copyright refers to the author's (creators of all sorts such as writers, photographers, artists, film producers, composers, and programmers) exclusive right to reproduce, prepare derivative works, distribute copies, and publicly perform and display their works. These rights may be transferred or assigned in whole or in part in writing by the author. Unless otherwise agreed in writing, work created by an employee is usually owned by the employer. The U.S. Copyright Act gets its authority from Article 1, Section 8, cl. 8 of the U.S. Constitution.
What is "fair use"?
Fair use is an exception to the exclusive protection of copyright under American law. It permits certain limited uses without permission from the author or owner. Depending on the circumstances, copying may be considered "fair" for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship or research.
To determine whether a specific use under one of these categories is "fair," courts are required to consider the following factors:
Keep in mind that even in an educational setting, it is not fair use to copy for a "commercial motive" or to copy "systematically," that is, "where the aim is to substitute for subscription or purchase." No factor by itself will determine whether a particular use is "fair." All four factors must be weighed together in light of the circumstances.
--from The Library of Congress, Copyright and Primary Sources