The concept of Karma is India’s unique contribution to the world. Hinduism, Buddhism and Jainism, the major religions of the world which originated in India, all acknowledge the universality of the law of karma in their own individual ways. According to Jainism, karma is not just a metaphysical law, but a real substance which flows into people and attaches itself to them like an impurity as they engage in various actions. People are born again and again until they rid themselves of the karmic substance.
According to Buddhism, Karma is an eternal law(read Buddha quotes on karma), which is responsible for the births and deaths and the suffering of beings in the causative world or samsara. While no one can really be free from the law of karma, people can minimize its negative impact by leading a righteous life, following the Eightfold Path. According to the three religions, the law of karma is applicable not only to humans but all beings, including plants, animals and microorganisms.
The early Vedic people were not familiar with the concept of karma. However they had an ethical sense and awareness of dharma (divine justice) and righteous actions. They believed that by pleasing the divinities and performing ritual acts in a prescribed manner, men could enter the higher worlds, by the path of the moon or that of the sun according to their deeds. It is difficult to say whether they believed in the rebirth or reincarnation of souls. Probably they did not.