A challenge is an attempt to remove or restrict materials, based upon the objections of a person or group.
A banning is the actual 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.
Throughout history, more and different kinds of people and groups of all persuasions than you might first suppose, who, for all sorts of reasons, have attempted—and continue to attempt—to suppress anything that conflicts with or anyone who disagrees with their own beliefs.