Reincarnation is believed to occur when the soul or spirit, after the death of the body, comes back to life in a newborn body. This phenomenon is also known as transmigration of the soul or metempsychosis.

This doctrine is a central tenet within the majority of Indian religious traditions, such as Hinduism, Jainism, and Sikhism; the Buddhist concept of rebirth is also often referred to as reincarnation. The idea was also fundamental to some Greek philosophers and religions as well as other religions, such as Druidism and later on, Spiritism, and Eckankar. It is also found in many small-scale societies around the world, in places such as Siberia, West Africa, North America, and Australia.

Although the majority of sects within Judaism, Christianity and Islam do not believe that individuals reincarnate, particular groups within these religions do refer to reincarnation; these groups include the mainstream historical and contemporary followers of Kabbalah, the Cathars, the Druze and the Rosicrucians. The historical relations between these sects and the beliefs about reincarnation that were characteristic of the Neoplatonism, Hermeticism, Manicheanism and Gnosticism of the Roman era, as well as the Indian religions, is unclear.

Source: http://en.wikipedia.org/wiki/Reincarnation