Heaven is a concept found in many world religions or spiritual philosophies that describes a blissful existence achieved after death. In the Western monotheist traditions (and also some other traditions? Valhalla?), heaven is a place of eternal happiness that can be achieved after death.
In Eastern religions (and some Western traditions), with their emphasis on reincarnation, heaven as such is less important. 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 again. Buddhism on the other hand has nirvana, but it is questionable how much the concept of nirvana has in common with that in heaven.
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 deities offer of salvation. In yet other religions (Calvinism, another variety of Christianity), those who go to heaven go, not because of anything they have done or even freely believed in, but because God has arbitrarily chosen to favour them by predestining them to go there.