The Avatar is the spirit of the planet incarnate in human form, and thus the only physical being with the ability to practice all four elements. It is considered the Avatar's duty to master the four elemental disciplines, and use such power to keep balance amongst the four nations of the world. With the death of the Avatar, the spirit is reborn into the following nation, dictated by the cyclic order; water, earth, fire, air. Coincidentally, the cycle of reincarnation corresponds to the cycle of the seasons; each of which is affiliated with a particular nation and element; water and winter, earth and spring, fire and summer, air and autumn. The cycle is also the traditional order in which any one Avatar is to master the elements, beginning at which ever element they were born into. The cycle can only be broken if the Avatar dies while using the Avatar State.
Each nation has a method of recognizing the new Avatar. Among the Air Nomads, this involved presenting all infants with a selection of toys to play with, and watching for a child that selected four specific toys that had belonged to past Avatars, called the four Avatar relics. Traditionally, the child is told of their identity as the Avatar on their sixteenth birthday, however, exceptions suchs as Avatars Aang and Korra exist. The Avatar possesses an innate connection to the Spirit World, and uses it as a bridge by which to reconcile the two worlds and ensure harmony. The Avatar is also capable of using this ability to contact the spirits of previous Avatars, and harness their combined energy to perform feats of bending beyond that of any other bender. Throughout the ages, countless incarnations of the Avatar have served to maintain harmony in the world.
Avatars also possess the ability to bend energy, though very few Avatars have learned this technique, and even fewer have used it. The Avatar is capable of bending more than one element at the same time.
When the Avatar masters control of the Avatar State, he or she is commonly said to be a 'Fully Realized Avatar'. However, it has not yet been confirmed whether mastery of the Four Elements, namely water, earth, fire and air are required for this status as well.
The reincarnations of the Avatar are controlled by the Avatar Cycle of the elements: water, earth, fire, air. The cycle is based on the passage of the four seasons (water is winter, earth is spring, fire is summer, and air is autumn).
When an Avatar dies, the next Avatar will be born into the next nation in the Avatar Cycle, for example, Avatar Yangchen from the Air Nomads was followed by Avatar Kuruk from the Northern Water Tribe.
The Avatar Cycle also determines the order in which each Avatar must learn the four bending disciplines. The Avatar begins by mastery of his or her native elements (so an Avatar born into the Water Tribe would first learn waterbending). The Avatar then goes on to learn the three remaining bending disciplines in the order of the cycle (the Water Tribe Avatar would learn first waterbending, then earthbending, then firebending, and finally airbending, in that order).