The Avatar is the human incarnation of the planet whose goal is to maintain balance in the world.
The Avatar is the incarnation of the planet who's main goal in life is to keep balance in the world. The Avatar spirit began when Wan and Raava fused together. Able to harnass the power of all four elements, the Avatar is also able to communicate with various spirits in the world and demons as well. When an Avatar dies and a new one is reborn, a cycle is followed to see where the next Avatar will be found: water, earth, fire, air. The next Avatars are Wan reincarnated. Tests are usually given to see who is the new Avatar such as checking when that person was born or having them select from a group of toys, a certain selection indicating the cycle of reincarnation (similar to the Dali Lama in Buddhism). The cycle was nearly destroyed in the Hundred Years War when the Fire Nation attempted to destroy the Air Nomads and Water Tribes to keep the Avatar from being reborn but ultimately failed. As a result though, new precautions have been taken to ensure the safety of the Avatar in the coming years.