Avatar: The Last Airbender takes place in a fantasy world, home to humans, fantastic animals, and supernatural spirits. Human civilization is divided into four nations, the Water Tribes, the Earth Kingdom, the Air Nomads, and the Fire Nation. Within each nation, an order of men and women called "Benders" have the ability to manipulate their native element. These Bending arts combine a certain style of martial arts and elemental mysticism. The Bending types are Waterbending, Earthbending, Firebending, and Airbending.
In each generation, one person is capable of Bending all four elements; this is the Avatar, the spirit of the planet manifested in human form. When the Avatar dies, he or she is reincarnated into an unborn baby native in the next of the four nations in the Avatar Cycle, which parallels the seasons: winter for water, spring for earth, summer for fire, and autumn for air. While legend holds that the Avatar must master the elements in order, starting with their native element, this can sometimes be compromised when the situation requires it. Learning to bend the element opposite one's native element is extremely difficult because opposing Bending arts are based on opposing fighting styles and doctrines. Firebending and Waterbending are opposites as are Earthbending and Airbending.
The Avatar possesses a unique power that resides within him or her, called the Avatar State. It is a defense mechanism that empowers the Avatar with the skills and knowledge of all the past Avatars. When the state is reached, the current Avatar will be able to wield the combined power of all past lives. When the Avatar enters this state, his or her eyes and mouth, and tattoos in the case of an Airbender, begin to glow. The glow is the representation of all the Avatar's previous incarnations focusing their energy through his or her body. However, if the Avatar is killed in the Avatar State, then the reincarnation cycle will be broken, and the Avatar will cease to exist.