Fair use is what maintains the balance between
copyright and the
First Amendment. Most simply, it requires that the public be able to copy/perform/display some of a work without getting permission from its
copyright holder. So the
film critic doesn't need authorization to take a clip of the movie she pans, and the rap artist doesn't need permission to make a
parody.
Fair use is an indefinite standard, codified at 17 U.S.C. 107 in a four-factored test. That means no one can say definitely that "copying 1 page is fair use but copying 10 is not" -- it all depends on the circumstances; copying an entire article may be fair for classroom use. Its vagueness is one of its virtues, however, leaving fair use adaptable to changing circumstances.
Fair use is under attack by the anticircumvention provisions of the DMCA. Yet even the Supreme Court itself has said that fair use cannot be determined mechanically: "[t]he task [of fair use analysis] is not to be simplified with bright-line rules, for the statute, like the doctrine it recognizes, calls for case-by-case analysis." Campbell v. Acuff-Rose Music, Inc., 510 U.S. 569, 577 (1994). (That case approved as fair use 2 Live Crew's song "Oh Pretty Woman", parodying Roy Orbison's "Pretty Woman.")