The Copyright Act gives authors of certain works (such as books, music, movies, etc.) the exclusive right to use those works. In other words—with one surprisingly large exception—nobody can use copyrighted works without permission of the author. The exception to the rule of exclusive right is fair use, a statutorily created exception which, in certain circumstances, allows anybody to use portions of a copyrighted work without permission and without having to compensate the author of the work.
The concept behind fair use is that it is reasonable to permit some limited use of copyrighted material to allow comment on, or criticism of, prior works. The determination of what is and is not a fair use requires a balancing of four factors. There is no objective, black and white test for fair use, and that leads to a substantial amount of litigation on the subject.