10 Big Myths about copyright explained. See EFF notes on fair use and links from it for a detailed answer, but bear the following in mind: 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. Paradise Lost: The Poem. Censorship: Wielding the Red Pen - 785 Dirty Words.