From Wikipedia: Fair use is a doctrine in United States copyright law that allows limited use of copyrighted material without requiring permission from the rights holders, such as use for scholarship or review. It provides for the legal, non-licensed citation or incorporation of copyrighted material in another author's work under a four-factor balancing test. It is based on free speech rights provided by the First Amendment to the United States Constitution. The term "fair use" is unique to the United States; a similar principle, fair dealing, exists in some other common law jurisdictions. Civil law jurisdictions have other limitations and exceptions to copyright."

What this means in practical terms is that you may, in certain circumstances, use non-licensed copyrighted materials (images, text, etc.) for criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research. Please read the Wikipedia entry for more information and a better description of the "four part balancing test" used to govern Fair Use principles.