Overview
In the books, it is a mysterious force that leads all living (and unliving) creatures. It is the will of Gan, the approximate equivalent of destiny or fate, in King's fictional language of High Speech. Ka can be considered to be a guide, a destination, but is certainly not a plan - at least, not one that is known to mortals. Ka is not necessarily a force of good or evil; it manipulates both sides, and seems to have no definite morality of its own. To this end, Ka resonates with the concept of Karma.
The official Dark Tower site (see below) describes ka as the following: "Ka... signifies life-force, consciousness, duty and destiny. In the vulgate, or low speech, it also means a place to which an individual must go."
Ka is a theme derived from the Egyptian model of the soul.
Read more about this topic: Ka (Dark Tower)