The doctrine of fair use was originally adopted by judges ruling in early copyright cases. Ultimately, Congress incorporated the doctrine into the Copyright Act of 1976, where fair use is now codified at Section 107 of Title 17 of the U.S. Code. In creating section 107, Congress listed four factors to be considered in determining whether a use is fair or not:
(1) the purpose and character of the use, including whether the use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
see also FairUseOnTheInternet