Challenge: an attempt to remove or restrict materials, based upon the objections of a person or group.
Ban: the removal of challenged materials.
Censorship: a change in the access status of material, based on the content of the work and made by a governing authority or its representatives.
Intellectual Freedom: the right of every individual to seek and receive information from all points of view without restriction.
Definitions are from ALA's "Researching Banned or Challenged Books" guide.
Banned Books Week is an annual event celebrating the freedom to read. Highlighting the value of free and open access to information, Banned Books Week brings together the entire book community –- librarians, booksellers, publishers, journalists, teachers, and readers of all types –- in shared support of the freedom to seek, to publish, to read, and to express ideas, even those some consider unorthodox or unpopular. By focusing on efforts across the country to remove or restrict access to books, Banned Books Week draws national attention to the harms of censorship. –- Banned Books Week Q&A