Avatar: The Last Airbender takes place in a world home to humans, hybrid animals, and spirits. Human civilization is divided into four nations: the Water Tribes, 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 and control the element of their nation using the physical motions of martial arts. The show's creators based each bending style on an existing martial art, leading to clear visual and physical differences in the techniques used by Waterbenders (T'ai chi ch'uan), Earthbenders (Hung Ga kung fu, for the most part), Firebenders (Northern Shaolin kung fu) and Airbenders (Baguazhang).[20] In addition to these four types of bending, there are several minor subcategories of bending within them, including but not limited to: Metalbending (Earth), Lightningbending (Fire) and Bloodbending (Water). Energybending is a lost, fifth form of bending that makes its only appearance in the series finale.

At any given time, there is only one person in the world capable of bending all four major elements: the Avatar. The Avatar is the divine spiritual entity of the world continuously reborn and reincarnated in human form. When an Avatar dies, this spirit/soul is reborn and reincarnated into the next nation in the Avatar Cycle, according to the implied correspondence of seasons to the nations' cultures (Winter/Water Tribe; Spring/Earth Kingdom; Summer/Fire Nation; and Autumn/Air Nomads) and must master each bending art in seasonal order, starting with their own native/birth element. Additionally, the Avatar possesses an immensely powerful spiritual ability called the Avatar State, which briefly endows them with all of the knowledge and abilities of all past Avatars as a self-triggeringdefense mechanism, which can be made subject to the will of the user by extensive trial and training in total chakra energy harmony. Being killed in the avatar state can cause the avatars to cease to continue.[21] If an Avatar is killed in the Avatar State, the reincarnation cycle is broken, and the Avatar identity will cease to exist.[22] Through the ages, the succeeding Avatars have served to keep a relative equality among the nations,[20] however, since the Avatar connects the material world to the spiritual world, and vice versa, the Avatar also must strive to keep balance between the worlds.[23]

Source: http://en.wikipedia.org/wiki/Avatar:_The...