The world of Avatar: The Last Airbender takes place in a world home to humans, fantastic animals, and spirits.
Human civilization is divided into four nations: the Water Tribe, the Earth Kingdom, the Fire Nation, and the Air Nomads.
Each nation has a distinct society, wherein people known as Benders have the ability to manipulate the eponymous element of their nation using the physical motions of martial arts.
At any given time, there is only one person alive capable of 'bending' all four elements: the Avatar, the spirit of the planet in human form.
When an Avatar dies, this spirit is reincarnated into the next nation in the Avatar Cycle, in the order of the seasons, and must master each bending art in seasonal order, starting with their native element. Additionally, the Avatar possesses an ability called the Avatar State, which briefly endows it with the knowledge and abilities of all past Avatars as a self-triggering defense mechanism, which can be made subject to the will of the user by extensive trial and training.
If an Avatar is killed in the Avatar State, the reincarnation cycle is broken, and the Avatar entity will cease to exist.
Through the ages, the succeeding Avatars have served to keep the four nations in harmony, and maintain world order. The Avatar serves as the bridge between the physical world and the spirit world, allowing each to solve problems that normal benders cannot.