Banned means that a school district/public library/whatever has removed the book so that it is unavailable for patrons because of objections to content, etc. A challenged book is one that has been challenged by someone because of content, whatever, but has not (yet) been removed from use/circulation. If you're really interested in the topic, www.ala.org, the American Library Association's website has tons of information on banned books and the freedom to read. Every year they sponsor a Banned Book Week and update the list of the year's most banned and challenged works. If your look around video sites, there's also a great video of the author of Perks speaking at a BBW event.