Class defines more about an adventurer than anything else. Class helps determine various statistics, including defenses and hit points, and it provides an adventurer with most of his or her powers.
Only rare nonplayer characters and monsters have classes. If a nonplayer character or a monster does have a class, the creature has only the class features, powers, and such that are included in the stat block. In other words, the creature functions as a full member of the class only if the creature’s stat block says it does.
Published in Rules Compendium, page(s) 61-62.