Censorship is the suppression of ideas and information that some individuals, groups, or government officials find objectionable or dangerous. Would-be censors try to use the power of the state to impose their view of what is truthful and appropriate, or offensive and objectionable, on everyone else. Censors pressure public institutions, like libraries, to suppress and remove information they judge inappropriate or dangerous from public access, so that no one else has the chance to read or view the material and make up their own minds about it. The censor wants to prejudge materials for everyone. It is no more complicated than someone saying, “Don’t let anyone read this book, or buy that magazine, or view that film, because I object to it!”
“Libraries should challenge censorship in the fulfillment of their responsibility to provide information and enlightenment.” — Article 3, Library Bill of Rights
The American Library Association (ALA) defines a banned book as a book that has been removed from the shelf of a library or school.
According to ALA, a challenged book is a book that a person, group, or authority thinks should be removed, but is still on a library or school's shelves.
ALA has a helpful list of frequently asked questions about banned books and lists of frequently challenged books which can be accessed here.