Fair use is a rule that allows a person to use copyrighted material without the permission of the copyright holder.
Copyright law tells us that when a person authors an original creative work he has certain rights. For example, if a person writes a book he will probably have the right to, among other things, prevent another person from copying the book or writing a sequel to the book. If another person wants to copy the book or write a sequel, the author of the book will need to give his permission first.
Fair use provides that a person might be able to use the copyrighted material without getting permission. Types of fair use include using copyrighted material for news, criticism, commentary, education, scholarship, and research. For example, a book critic or a teacher could almost certainly quote or copy portions of the book without permission of the author.
Courts decide what is fair use and what is not fair use. Judges apply a number of factors to determine whether use of the copyrighted material is really fair use. Among other things, a court will look at the nature of the copyrighted material, how the copyrighted material was used, how much of the copyrighted material was used, and whether the holder of the copyright will be financially damaged by the use of his copyrighted material. Here’s a video on fair use: