Fair use is the key limitation on the copyright holder's exclusive rights to a work.

Fair use is a provision of US Copyright law, Section 107, that allows for limited use of copyrighted materials for certain individuals (such as professors or reporters) and under certain circumstances without permission from the rights' holders. Circumstances that constitute fair use include works being used for the purposes of:

criticism, commentary, news reporting, teaching, scholarship and research.

Section 107 includes FOUR FACTORS that are to be considered each time one wants to make use of a copyrighted material. The four factors include:

1) the purpose and character of the use, including whether such use is for nonprofit educational purposes or of a commercial nature;
2) the nature of the copyrighted work;
3) the amount copied and the substantiality of the portion used in relation to the copyrighted work as a whole; and
4) the effect of the use upon the potential market for, or value of, the copyrighted work

For more information, see Section 107 of the US. Copyright law which covers Limitations on exclusive rights of the copyright holder, or the University of Texas System's Copyright Rules of Thumb on fair use.