A book which contains bitchy comments about other people, often friends, school mates, etc.

When you eat dirt for the first time and leave your family to build a shack in the woods

See Urban Dictionary.

a book where girls write bitchy, rude, horrible things about other girls that may or may not be true.

A site which contains bitchy comments about other people, including but not limited to friends, peers, celebrities, politicians, etc. An online burnbook as the name references does not need to be a physical book but can be a website such as burnbook.com.

(Taken from Mean Girls) A profane book used to describe people that you have utmost distaste for. Contains a picture, name, and reasons for their entry.

this is a burn book. urban dictionary has became a burn book if you haven't noticed. only worldwide. pff