A karaka is a significator: the planet that 'does' a thing in a chart. There are three senses and they must not be confused. The naisargika (natural) karaka is the fixed signification of a graha - Jupiter for children, Venus for marriage. The bhava karaka is the fixed planet of a house - the Sun for the 1st, Jupiter for the 2nd. The chara (variable) karaka is the Jaimini scheme in which the eight karakas are assigned by degree within a sign, the planet with the highest degree being the atmakaraka, the one of the self.