Banning books is fundamentally a form of censorship used to limit the availability/access of a book from a specific group of people or the public. Censorship — the suppression of ideas and information — can occur at any stage or level of publication, distribution, or institutional control. (ALA.org)

This is not an accidental action. Banning a book is a process where first the book must be challenged. Challenging a book is when a person/group/organization attempts to limit or remove the access of a book.  According to the ALA (American Library Association), most books are challenged based on a desire to protect children from sexually explicit, offensive language and/or because they are deemed unsuited to any age group. If a governing power agrees with the challenge, they can choose to limit the availability of the book. This could include removing it from school curriculums and/or library shelves.

Thankfully most challenges are unsuccessful, and most materials are retained in the school curriculum or library collection because of committed parents, librarians, community members, teachers and students.