Karma is the action, the result of the action is called
karma-vipaka.
Things are not causeless (ahetuka) nor due to one single cause
(eka-hetuka).
According to Buddhism, man is conditioned by his biological laws
(bijaniyama), by his environment and physical laws (utu-niyama), by
psychological laws (citta-niyama) including his karmic heritage (kamma-niyama);
he is not determined by any or all of them.
It has an element of free will (attakara) or personal endeavor
(purisakara) by exercising which, he can change his own nature as well as his
environment (by understanding it) for the good of himself as well as others.
Karmic process (karmabhava) is the energy that out of a present
life, conditions a future life in unending sequence.
The last thought-moment in this life conditions the first-thought
moment in the next.
The craving for re-existence makes him re-exist.
What you call life here is the functioning of the five aggregates
namely: material form, feelings, perception, mental formations or dispositions
and consciousness.
One may ask: “If every death is followed by a birth, the world’s
population should be constant, but how is it that, everyone knows, the world’s
population is fast increasing year by year?” Rebirth can take place not only in
this world (whose population only we can count) but in other world systems of
which the Buddhist texts speak.
The dying individual with his whole being convulsively clinging to
life, at the very moment of his death, sends forth karmic energies which like
flash of lightning, hit a new mother’s womb.
