Before we explore the different types of censorship impacting libraries, it is important that we describe what types exist and how they impact libraries and their communities:
"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. "
Source: ALA's page on banned & challenged books, https://www.ala.org/bbooks/banned
Why are books challenged/banned?
Books are challenged, often, with good intentions. The goal is to protect others, particularly minors, from difficult ideas and information (American Library Association). Titles that were challenged are often deemed "sexually explicit" or contain "offensive language." Sometimes, materials are deemed "unsuited to any age group."