WHAT IS KAMMA?
"Volition is Kamma."
-- ANGUTTARA NIKĀYA
The Pāli term Kamma, literally, means action or doing. Any kind of intentional action whether mental, verbal, or physical is regarded as Kamma. It covers all that is included in the phrase: "Thought, word and deed". Generally speaking, all good and bad actions constitute Kamma. In its ultimate sense Kamma means all moral and immoral volition (kusala akusala cetanā). Involuntary, unintentional or unconscious actions, though technically deeds, do not constitute Kamma, because volition, the most important factor in determining Kamma, is absent. 
The Buddha says: -- "I declare, O Bhikkhus, that volition (cetanā) is Kamma. Having willed one acts by body, speech and thought."
Every volitional action of persons, except those of Buddhas and Arahants, is called Kamma. An exception is made in their case because they are delivered from both good and evil. They have eradicated both ignorance and craving, the roots of Kamma. "Destroyed are their (germinal) seeds (khīna-bijā), selfish desires no longer grow," states the Ratana Sutta. This does not mean that the Buddhas and Arahants are passive. They are tirelessly active in working for the real well-being and happiness of all. Their deeds, ordinarily accepted as good or moral, lack creative power as regards themselves. Understanding things as they truly are, they have finally shattered their cosmic fetters -- the chain of cause and effect.
Some religions attribute this unevenness to Kamma, but they differ from Buddhism when they state that even unintentional actions should be regarded as Kamma.
According to them, "the unintentional murderer of his mother is a hideous criminal. The man who kills or who harasses in any way a living being without intent, is none the less guilty, just as a man who touches fire is burnt. "
"This astounding theory undoubtedly leads to palpable absurdities.
"The embryo and the mother would both be guilty of making each other suffer. Further the analogy of the fire is logically fallacious. For instance, a man would not be guilty if he got another person to commit the murder, for one is not burnt if one gets another to put his hand into the fire. Moreover unintentional actions would be much worse than intentional wrong actions, for, according to the comparison, a man who touches fire without knowing that it would burn is likely to be more deeply burnt than the man who knows.
In the working of Kamma its most important feature is mind. All our words and deeds are coloured by the mind or consciousness we experience at such particular moments. "When the mind is unguarded, bodily action is unguarded; speech also is unguarded; thought also is unguarded. When the mind is guarded, bodily action is guarded; speech also is guarded; and thought also is guarded. "'
"By mind the world is led, by mind is drawn:
And all men own the sovereignty of mind."
"If one speaks or acts with a wicked mind, pain follows one as the wheel, the hoof of the draught-ox. "
"If one speaks or acts with a pure mind, happiness follows one as the shadow that never departs. "
Immaterial mind conditions all Kammic activities.
Kamma does not necessarily mean past actions. It embraces both past and present deeds. Hence, in one sense, we are the result of what we were, we will be the result of what we are. In another sense, it should be added, we are not totally the result of what we were, we will not absolutely be the result of what we are. The present is no doubt the offspring of the past and is the parent of the future, but the present is not always a true index of either the past or the future -- so complex is the working of Kamma. For instance, a criminal today may be a saint tomorrow, a good person yesterday may be a vicious one today.
It is this doctrine of Kamma that the mother teaches her child when she says: "Be good and you will be happy and we will love you. But if you are bad, you will be unhappy and we will not love you."
Like attracts like. Good begets good. Evil begets evil. This is the law of Kamma.
In short Kamma is the law of cause and effect in the ethical realm, or as some Westerners prefer to say, "action influence."
Kamma and Vipāka
Kamma is action, and Vipāka, fruit or result, is its reaction. Just as every object is accompanied by a shadow, even so every volitional activity is inevitably accompanied by its due effect. Like potential seed is Kamma. Fruit, arising from the tree, is the Vipāka, effect or result. As Kamma may be good or bad, so may Vipāka, fruit, be good or bad. As Kamma is mental, so Vipāka too is mental; it is experienced as happiness or bliss, unhappiness or misery according to the nature of the Kamma seed. Ānisamsa are the concomitant advantageous material conditions, such as prosperity, health and longevity.
When Vipāka's concomitant material conditions are disadvantageous, they are known as ādinava (evil consequences), and appear as poverty, ugliness, disease, short life span and the like.
By Kamma are meant the Moral and Immoral types of mundane consciousness (kusala akusala lokiya citta), and by Vipāka, the resultant types of mundane consciousness (lokiya vipākacitta).
According to Abhidhamma,  Kamma constitutes the twelve types of immoral consciousness, eight types of moral consciousness pertaining to the Sentient Realm (kāmāvacara), five types of moral consciousness pertaining to the Realms of Forms (rūpāvacara), and four types of moral consciousness pertaining to the Formless Realms (arūpāvacara).
The eight types of supramundane (lokuttara) consciousness are not regarded as Kamma, because they tend to eradicate the roots of Kamma. In them the predominant factor is wisdom (pa257;) while in the mundane it is volition (cetanā).
The nine types of moral consciousness pertaining to the Realms of Form and the Formless Realms are the five Rūpāvacara and four Arūpāvacara Jhānas (Ecstasies) which are purely mental.
Words and deeds are caused by the first twenty types of mundane consciousness. Verbal actions are done by the mind by means of speech. Bodily actions are done by the mind through the instrument of the body. Purely mental actions have no other instrument than the mind.
These twenty-nine  types of consciousness are called Kamma because they have the power to produce their due effects quite automatically, independent of any external agency.
Those types of consciousness which one experiences as inevitable consequences of one's moral and immoral thoughts are called resultant consciousness pertaining to the Sentient Realm. The five types of resultant consciousness pertaining to the Realms of Form and the four types of resultant consciousness pertaining to the Formless Realms are called Vipāka or fruition of Kamma.
As we sow, so we reap somewhere and sometime, in this life or in a future birth. What we reap today is what we have sown either in the present or in the past.
The Samyutta Nikāya  states:
"According to the seed that's sown,
So is the fruit ye reap therefrom
Doer of good (will gather) good.
Doer of evil, evil (reaps).
Sown is the seed, and planted well.
Thou shalt enjoy the fruit thereof."
Kamma is a law in itself which operates in its own field without the intervention of any external, independent ruling agency.
Inherent in Kamma is the potentiality of producing its due effect. The cause produces the effect, the effect explains the cause. The seed produces the fruit, the fruit explains the seed, such is their relationship. Even so are Kamma and its effect.
"The effect already blooms in the cause."
Happiness and misery, which are the common lot of humanity, are the inevitable effects of causes. From a Buddhist standpoint they are not rewards and punishments, assigned by a supernatural, omniscient ruling power to a soul that has done good or evil. Theists who attempt to explain everything by this one temporal life and an eternal future life, ignoring a past, may believe in a post-mortem justice, and may regard present happiness and misery as blessings and curses conferred on his creation by an omniscient and omnipotent Divine Ruler, who sits in heaven above controlling the destinies of the human race. Buddhism that emphatically denies an arbitrarily created immortal soul, believes in natural law and justice which cannot be suspended by either an Almighty God, or an All-compassionate Buddha. According to this natural law, acts bring their own rewards and punishments to the individual doer whether human justice finds him or not.
Some there are, who cavil thus: So you Buddhists too administer the opium of Kammic doctrine to the poor, saying:
"You are born poor in this life on acount of your past evil Kamma. He is born rich on account of his past good Kamma. So be satisfied with your humble lot, but do good to be rich in your next life.
"You are being oppressed now because of your past evil Kamma. That is your destiny. Be humble and bear your sufferings patiently. Do good now. You can be certain of a better and happier life after death."
The Buddhist doctrine of Kamma does not expound such fatalistic views. Nor does it vindicate a post-mortem justice. The All-merciful Buddha, who had no ulterior selfish motives, did not teach this law of Kamma to protect the rich and comfort the poor by promising illusory happiness in an after-life.
According to the Buddhist doctrine of Kamma, one is not always compelled by an iron necessity, for Kamma is neither fate nor predestination imposed upon us by some mysterious unknown power to which we must helplessly submit ourselves. It is one's own doing reacting on oneself, and so one has the power to divert the course of Kamma to some extent. How far one diverts it, depends on oneself.
The Cause of Kamma
Ignorance (avijjā) or not knowing things as they truly are, is the chief cause of Kamma. Dependent on ignorance arise Kammic activities (avijjā paccaya samkhārā), states the Buddha in the Paticca Samuppāda (Dependent Origination).
Associated with ignorance is its ally craving (tanhā), the other root of Kamma. Evil actions are conditioned by these two causes.
All good deeds of a worldling (puthujjana), though associated with the three wholesome roots of generosity (alobha), goodwill (adosa) and knowledge (amoha), are nevertheless regarded as Kamma because the two roots of ignorance and craving are dormant in him. The moral types of supramundane Path consciousness (maggacitta) are not regarded as Kamma because they tend to eradicate the two root causes.
The Doer of Kamma
Who is the doer of Kamma? Who reaps the fruit of Kamma? "Is it a sort of accretion about a soul?"
In answering these subtle questions, Venerable Buddhaghosa writes in the Visuddhi Magga:
"No Doer is there who does the deed,
Nor is there one who feels the fruit,
Constituent parts alone roll on,
This indeed is right discernment. 
According to Buddhism there are two realities -- apparent and ultimate. Apparent reality is ordinary conventional truth (sammuti sacca). Ultimate reality is abstract truth (paramattha sacca).
For instance, the table we see is apparent reality. In an ultimate sense the so-called table consists of forces and qualities.
For ordinary purposes a scientist would use the term water, but in the laboratory he would say H2O.
In the same way, for conventional purposes such terms as man, woman, being, self and so forth are used. The so-called fleeting forms consist of psycho-physical phenomena which are constantly changing, not remaining for two consecutive moments the same.
Buddhists therefore do not believe in an unchanging entity, in an actor apart from action, in a perceiver apart from perception, in a conscious subject behind consciousness.
Who then is the doer of Kamma? Who experiences the effect?
Volition or will (cetanā) is itself the doer. Feeling (vedanā) is itself the reaper of the fruits of action. Apart from these pure mental states (suddhadhammā) there is none to sow and none to reap.
Just as, says the Venerable Buddhaghosa, in the case of those elements of matter that go under the name of tree, as soon as at any point the fruit springs up, it is then said the tree bears fruit or "thus the tree has fructified," so also in the case of "aggregates" (khandhas) which go under the name of Deva or man, when a fruition of happiness or misery springs up at any point, then it is said "that Deva or man is happy or miserable."
In this respect Buddhists agree with Prof. William James when, unlike Descartes, he asserts: "Thoughts themselves are the thinkers. "
Where is Kamma?
"Stored within the psyche," writes a certain psychoanalyst, "but usually inaccessible and to be reached only by some, is the whole record, without exception, of every experience the individual has passed through, every influence felt, every impression received. The subconscious mind is not only an indelible record of individual experiences but also retains the impress of primeval impulses and tendencies, which so far from being outgrown as we fondly deem them in civilized man, are subconsciously active and apt to break out in disconcerting strength at unexpected moments."
A Buddhist would make the same assertion with a vital modification. Not stored within any postulatory "psyche", for there is no proof of any such receptacle or store-house in this ever-changing complex machinery of man, but dependent on the individual psycho-physical continuity or flux is every experience the so-called being has passed through, every influence felt, every impression received, every characteristic -- divine, human, or brutal-- developed. In short the entire Kammic force is dependent on the dynamic mental flux (citta santati) ever ready to manifest itself in multifarious phenomena as occasion arises.
"Where, Venerable Sir, is Kamma?" King Milinda questioned the Venerable Nāgasena.
'"O Mahārāja," replied the Venerable Nāgasena, "Kamma is not said to be stored somewhere in this fleeting consciousness or in any other part of the body. But dependent on mind and matter it rests manifesting itself at the opportune moment, just as mangoes are not said to be stored somewhere in the mango tree, but dependent on the mango tree they lie, springing up in due season. "
Neither wind nor fire is stored in any particular place, nor is Kamma stored anywhere within or without the body.
Kamma is an individual force, and is transmitted from one existence to another. It plays the chief part in the moulding of character and explains the marvellous phenomena of genius, infant prodigies, and so forth. The clear understanding of this doctrine is essential for the welfare of the world.