Heaven is a concept found in many world religions or spiritual philosophies that describes a place "not of this world". Generally, one cannot travel there unless one dies; although, according to some religions, exceptions have been made. Various religions have described heaven as being populated by angels, demons, gods and goddesses, and/or heroes. Heaven is generally construed as a place of eternal happiness. The relationship between this religious concept and the celestial sphere is generally believed to have been first proposed by the ancient astronomer-priests (see also: astrologer).
In Eastern religions (and some Western traditions), with their emphasis on reincarnation, the concept of heaven is not as prominent. But it still is present: for example, in Buddhism there are several heavens, and those who accumulate good karma will be reborn in a heaven; however their stay in the heaven is not eternal -- eventually they will use up all their good karma and be reincarnated as a human.
Religions which have a heaven differ on how one gets into it. Some (followers of universalism) provide that everyone will go to heaven, no matter what they have done on earth. Others make entrance to heaven conditional on having lived a "good life" (within the terms of the spiritual system); those who do not meet the criteria go to a place of punishment, hell. Other religions (many varieties of Christianity) make entrance to heaven conditional not on good works, but on having believed and trusted in the deity, and accepting the deity's offer of salvation. In yet other religions (Calvinism, a Protestant form of Christianity), those who go to heaven go, not because of anything they have done or independently chosen, but because God has chosen to favour them by predestining them to go there.
Heaven is an especially interesting doctrine in Christian thought, as the afterlife depends largely on the resurrection of the body. While the intermediate state (between death and the return of Christ) is unclear, the final state of believers is in a resurrected body, living in the "New Jerusalem" in the "New Earth." The person was never meant to be disembodied. Death is an enemy, not a friend who frees the soul.
The present Roman Catholic teaching regarding Heaven is found in the Catechism of the Catholic Church: "Those who die in God's grace and friendship and are perfectly purified live forever... This perfect life with [God]....is called heaven. [It] is the ultimate end and fulfilment of the deepest human longings, the state of supreme, definitive happiness." Pope John Paul II has said (see link below), "[Heaven] is neither an abstraction nor a physical place in the clouds, but a living, personal relationship with [God]."
The concept of heaven is well-defined within the Christian and Islamic religions. The Jewish concept of the afterlife is sometimes known as "olam haba", the world to come, but Judaism's afterlife beliefs were never set forth in a systematic or official fashion as was done in Christianity and Islam.