An NPC class functions exactly like a base class, except that they are only appropriate for non-player characters or for extremely low power games where the normal classes aren’t used.
A character with levels in an NPC class cannot take levels in any other class. If they gain levels in a PC class or a different NPC class, then they lose all their previous NPC class levels and gain a number of levels in their new class equal to half their original class level, rounded down to a minimum of 1.