With respect to how it manifests in our lives, there are four kinds of karma. First is ripening or fruition karma; next is resultant karma; then there is remaining karma or karma that has not come to a conclusion yet; and finally, there is simultaneous karma, in which the result immediately occurs with the action. Where do we stand in relationship to these dimensions of karma? At any given moment in our life we are really not sure for example, whether we are experiencing resultant karma. We are not sure to what extent we are creating new karma, whether our actions have any lasting results or residues, nor do we understand simultaneous karma. I will not try to go into all of the four kinds of karma, but for our purpose today fruition karma is most important.
Now we will talk about fruition karma, or the ripening of karma. With karma-as-intention the karma created is not as great as the karma from actually doing it. On the other hand once our thought turns into karma-as-intention-manifested, the real word repercussions will be greater and karmic retribution from that action will also be greater .'Retribution 'in relation to karma carries a neutral meaning, as it depends on the kinds of cause and the kind of results.
Karma can ripen in a three ways: through thought through speech, and through action. Karma-as-intention that does not ripen into karma-as-intention-manifested is ‘concealed karma,’ since it only exists in one’s mind .Opposed to this is ‘manifest karma,’ which refer to karma-as-intention plus speech and/or action. Relating this to intention, we see that karma can ripen as intention only, intention plus speech, and intention plus action.
Does just thinking about killing someone create dab karma? When you understand karma as cause and effect, you will see that evil thoughts indeed accumulate karma. Merely by dwelling on the idea of killing someone, you put into motion a causal relation with repercussions. These kinds of thoughts constitute one’s mental life, and if there is sufficient accumulation they can manifest in speech or action. In the sutras the Buddha says that in the world of samsara there is not a single action or even giving rise to thought that is excluded from creating karma. Whatever sentient beings do or think is centered on attachment to self, and because of this they continue to create karma. Therefore, when we even have thoughts of killing, as Buddhist practitioners we should give rise to a sense of contrition and practice repentance.
Generally speaking, when one engages in negative actions (the cause), one will reap negative results (the effect). This is the causal result of bad karma. Correspondingly, when one engages in virtuous actions, one will reap virtuous results. This is the causal result of virtuous karma. There is another kind of karma that is neither good nor bad, and I'll get to that later. Speaking generally however, karma can be good, bad, or neutral.
With karma that is neutral the determining factor is the state of mind while engaging in such actions, whether there are subtle leanings towards wholesome or unwholesome. There will still be retribution tilted towards good or bad, but it will be mild. However, there are genuinely neutral karmic actions, with neither wholesome nor unwholesome overtones and the retribution that results will be neither good nor bad.
Depending on their karma sentient beings can be reborn in one of six modes or realms2 of existence. A sentient being that engages in wholesome actions will receive retribution by being reborn in one of the three upper realms--the human realm, or one of the two heavenly realms. A sentient being that engages in unwholesome actions will receive retribution by being reborn in one of the three lower realms--the animal realm, the realm of angry spirits, or most severely the hell realm. Thus one's accumulated karma determines where among the six realms, and what form they will take at the next rebirth.
Still another twofold division is made according to the practice of the path: karma with vexations and pure karma.
Karma with vexations includes good, bad, and neutral karma, and is the origin of suffering. Pure karma is created by the practice of Buddhadharma, the path leading out of suffering. Engaging in pure karma, one can become free from the origin of suffering.
Source : "Setting in Motion the Dharma Wheel"