A challenge is an attempt to remove or restrict materials, based upon the objections of a person or group. A banning is the removal of those materials. Challenges do not simply involve a person expressing a point of view; rather, they are an attempt to remove material from the curriculum or library, thereby restricting the access of others. Due to the commitment of librarians, teachers, parents, students and other concerned citizens, most challenges are unsuccessful and most materials are retained in the school curriculum or library collection.
Books usually are challenged with the best intentions—to protect others, frequently children, from difficult ideas and information. Often challenges are motivated by a desire to protect children from “inappropriate” sexual content or “offensive” language. The following were the top three reasons cited for challenging materials as reported to the Office of Intellectual Freedom:
Although this is a commendable motivation, Free Access to Libraries for Minors, an interpretation of the Library Bill of Rights (ALA's basic policy concerning access to information) states that, “Librarians and governing bodies should maintain that parents—and only parents—have the right and the responsibility to restrict the access of their children—and only their children—to library resources.” Censorship by librarians of constitutionally protected speech, whether for protection or for any other reason, violates the First Amendment.
Taken from: About Banned and Challenged Books; http://www.ala.org/bbooks/about