UTF-7 does map a sequence of code points to a sequence of bytes like the other UTFs but unlike them there is more than one valid way to represent a peice of text in UTF-7 and its output is designed to be used directly in internet mail. Essentially it was a case of recognising that UTF-8+quoted printable=insanely inefficiant encoding and doing it better by designing a single process for the entire task. 
From a registration and mail header point of view UTF-7 is considered to be a character set (e.g. its listed at http://www.iana.org/assignments/character-sets).