Karma implies `action` or `doing`; whatever one does, says, or thinks is a karma.

In Buddhism, the term karma is used specifically for those actions which spring from:
1. mental intent
2. mental obsessions

which bring about a fruit (Pali, phala) or result (vipaka), either in the present life, or in the setting of a future rebirth. Karma is the engine which drives the wheel of the cycle of uncontrolled rebirth (samsara) for each being.

In the (Anguttara Nikaya Nibbedhika Sutta) the Buddha said :

"Intention (cetana), monks, is kamma, I say. Having willed, one acts through body, speech and mind".

Every time a person acts, there is some character of intent at the bottom of the mind and it is that quality rather than the outward appearance of the action that determines the effect. If a person professes piousness and virtue but still acts with greed, anger or hatred (veiled behind an outward display of well-meaning intent) then the fruit of those actions will bear proof to the underlying intention that lay behind them and will be a cause for future unhappiness. The Buddha spoke of wholesome actions (kusala-kamma) -- that result in happiness, and unwholesome actions (akusala-kamma) -- that result in unhappiness.

Karma is thus used as an ethical principle and a cosmological explanation for the world. Buddhists are of the faith that the actions of beings decide their own future, and due to this there are no private actions - every action has a consequence. The accent of karma in Buddhism is on mindful action, not on faulting someone else for whatever happens to oneself.

There is a further distinction between worldly, wholesome karma that leads to sams?ric happiness (like birth in higher realms), and path-consciousness which leads to enlightenment and (nirvana). Thus, there is sams?ric good karma, which leads to temporal happiness, and then there is liberating karma - which is superlatively good, as it terminates suffering forever. Once one has achieved liberation one does not yield any further karma, and the corresponding states of mind are called in Pali as Kiriya. Nevertheless, the Buddha advocated the practise of wholesome actions: "Refrain from unwholesome actions/Perform only wholesome ones/Purify the mind/This is the teaching of the Enlightened Ones."

In Buddhism, the term karma is often used to refer only to samsaric karma, as indicated by the twelve nidanas of dependent origination.

Due to the inevitability of consequence, Karma entails the belief of Buddhist rebirth. However, karma is not the only basis of rebirth. The rebirths of eighth stage (and above) Bodhisattvas in the Mahayana tradition refers to those liberated beings who consciously prefer to be reborn in a future life in order to help others still entrapped in sams?ra. However, this is not `uncontrolled` rebirth anymore.

The Buddha explains what it means to have conviction in karma: First, karma happens in reality -- it is not a mere illusion. Second, one really is responsible for one`s actions. There is no external force like the stars or some good or evil being acting through oneself. When one is conscious, he/she is the one who decides what happens. Third, one`s actions are bound to bring results - one is not just writing on water -- and those results can be good or bad, depending on the quality of the purpose behind the act.