Well... There're four nations - the Water Tribes, the Earth Kingdom, the Fire Nation, and the Air Nomads, which I belong to.
Each generation, the Avatar is born into one of them, following a cycle like the one of the seasons. The Avatar is meant to keep balance and harmony in the world, not only between spirits and humans but between all nations as well.
The Avatar is the only one who can master all four elements, and access the Avatar State, when we link to all our past lives and obtain a lot of power we wouldn't normally have.