Lots of people have mentioned Fair Use in this thread. Not that I feel anyone is incorrect in what they've said about it, but it is best if people go straight to the source of Fair Use information, rather than going by hearsay.
Fair Use is defined in the Copyright Law of the United States of America and Related Laws Contained in Title 17 of the United States Code - Section 107 Limitations on exclusive rights: Fair use
and interpreted on the U.S government's copyright web site Fair Use
Go to the source.