Fair use is part of U.S. Copyright Law and allows for limited use of copyrighted work without permission for purposes such as criticism, teaching, or research. Whether a use is considered fair depends on factors like how much of the work is used, the impact on the original work's market, and the purpose of the use. Fair use is determined on a case-by-case basis considering all factors, and differs from plagiarism by addressing use of copyrighted rather than non-copyrighted works.