Fair use doctrine provides a set of guidelines pursuant to which researchers, educators, scholars, and others may use copyrighted works without seeking permission or paying royalties. Fair use doctrine does not provide a right to use somebody else's work, but presents a defense against accusations of copyright violation for people who reasonably believed that their use of a copyrighted work was fair use. That means that if your use is challenged, you will have the burden of proving that your use qualified as "fair use".As a general rule, a copyright owner has the legal right to restrict the reproduction of a copyrighted work, and to demand royalties when copyrighted work is reproduced. The penalties for unauthorized reproduction of copyrighted work can be substantial. Accordingly, it makes sense to have a pretty good idea of whether or not you can defensibly copy part or all of somebody else's work without that person's permission, before violating copyright law.
Unfortunately, the determination of whether any given use is "fair use" can be complex. When evaluating whether or not the use of a copyrighted work is "fair use", there are four factors which must be considered:
What is the character of the use?
What is the nature of the work to be used?
How much of the work will you use?
What effect would this use have on the market for the original or for
permissions if the use were widespread?
As you may have inferred, fair use is not determined by a series of simple "yes or no" questions, but involves a relatively complex balancing test between the private interest of the copyright holder and the public interest. No particular use, no matter how much in the public interest or how educational, will automatically qualify as "fair use".
READ MORE: http://www.expertlaw.com/library/intellectual_property/fair_use.html