The Pali term Karma literally means action or doing. Any kind of intentional action whether mental, verbal, or physical, is regarded as Karma. It covers all that is included in the phrase "thought, word and deed". Generally speaking, all good and bad action constitutes Karma
Karma 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 present 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 Karma.
Karma is action, and Vipaka, 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. Karma is like potential seed: Vipaka could be likened to the fruit arising from the tree the effect or result. Anisamsa and Adinaya are the leaves, flowers and so forth that correspond to external differences such as health, sickness and poverty these are inevitable consequences, which happen at the same time. Strictly speaking, both Karma and Vipaka pertain to the mind.
As Karma may be good or bad, so may Vipaka, - the fruit is good or bad. As Karma is mental so Vipaka is mental (of the mind). It is experienced as happiness, bliss, unhappiness or misery, according to the nature of the Karma seed.
When Vipakas concomitant material things are disadvantageous, they are known as Adinaya, full of wretchedness, and appear as poverty, ugliness, disease, short life-span and so forth.
As we sow, we reap somewhere and sometime, in his life or in a future birth. What we reap today is what we have sown either in the present or in the past.
Karma is a law in itself, which operates in its own field without the intervention of any external, independent ruling agency. Thats why it's known as universal law.