According to the Buddha, our willed actions produce effects. They eventually return to ourselves. One effect is the immediately visible psychological effect. The other is the effect of moral retribution.
Firstly let us deal with the psychological effect of kamma. When a willed action is performed it leaves a track in the mind, an imprint which can mark the beginning of a new mental tendency. It has a tendency to repeat itself, to reproduce itself, somewhat like a protozon, like an amoeba. As these actions multiply, they form our character. Our personality is nothing but a sum of all our willed actions, a cross-section of all our accumulated kamma. So by yielding first in simple ways to the unwholesome impulses of the mind, we build up little by little a greedy character, a hostile character, an aggressive character or a deluded character. On the other hand, by resisting these unwholesome desires we replace them with their opposites, the wholesome qualities. Then we develop a generous character, a loving and a compassionate personality, or we can become wise and enlightened beings. As we change our habits gradually, we change our character, and as we change our character we change our total being, our whole world. That is why the Buddha emphasizes, so strongly the need to be mindful of every action, of every choice. For every choice of ours has a tremendous potential for the future.
Now let us examine the effects of moral retribution. What is most important in Kamma is its tendency to ripen in the future and produce results in accordance with the universal moral law.
Whenever we perform an action with intention, such action deposits a "seed" in the mind, a seed with a potency to bring about effects in the future. These effects correspond to the nature of the original action. They follow from the inherent ethical tone of the action. Our unwholesome kamma comes back to us and lead to our harm and suffering. Our wholesome kamma eventually returns to us and leads to our happiness and well being.
Seen from this angle, from the standpoint of karmic law, the universe appears to maintain a certain moral equilibrium, a balance between all the morally significant deeds and the objective situations of those who perform them. So the law of kamma is a moral application of the general principle that for every action there is an equal and an opposite reaction. However, the working of kamma is not mechanical. Kamma is willed action and the kamma is something alive and organic. Therefore kamma allows much room for variation, for the play of living forces.