Copyright & Fair Use Explained

"Fair use is a copyright principle based on the belief that the public is entitled to freely use portions of copyrighted materials forpurposes of commentary and criticism. For example, if you wish to criticize a novelist, you should have the freedom to quote a portion of the novelist's work without asking permission. Absent this freedom, copyright owners could stifle any negative comments about their work.

Unfortunately, if the copyright owner disagrees with your fair use interpretation, the dispute will have to be resolved by courts or arbitration. If it's not a fair use, then you are infringing upon the rights of the copyright owner and may be liable for damages.

The only guidance is provided by a set of fair use factors outlined in the copyright law. These factors are weighed in each case to determine whether a use qualifies as a fair use. For example, one important factor is whether your use will deprive the copyright owner of income. Unfortunately, weighing the fair use factors is often quite subjective. For this reason, the fair use road map is often tricky to navigate."

This summarty from Stanford University Libraries explains the various rules behind the fair use principle. Found via this surf11.com post.