This is one of the most confusing things in copyright law for the lay person but I will try and explain it here.
The "fair use" exemption to (U.S.) copyright law was created to allow things such as commentary, parody, news reporting, research and education about copyrighted works without the permission of the author. That's vital so that copyright law doesn't block your freedom to express your own works -- only the ability to appropriate other people's. Intent, and damage to the commercial value of the work are important considerations.
Links