The 4xx codes are intended for cases in which the client seems to have erred, and the 5xx codes for the cases in which the server is aware that the server has erred. It is impossible to distinguish these cases in general, so the difference is only informational.
The body section may contain a document describing the error in human readable form. The document is in MIME format, and may only be in text/plain, text/html or one for the formats specified as acceptable in the request.