The Avatar, formed initially when Wan permanently fused with Raava during the Harmonic Convergence of 9,829 BG, is the human embodiment of light and peace through the connection with the Avatar Spirit. As the only physical being with the ability to bend all four elements, it is considered the Avatar's duty to master the four Bending arts and use that power to keep balance among the four nations of the world, as well as between mankind and spirits. Upon death, the Avatar Spirit causes the Avatar to reincarnate into the next nation, dictated by the cyclic order: fire, air, water, and earth. Mirroring the order in which the first Avatar, Wan, learned the elements, this is also the traditional order in which any one incarnation is to master the elements, beginning with the bending art of the Avatar's birth nation. The reincarnation cycle can only be broken if the Avatar is killed while in the Avatar State, Raava is removed from the Avatar and destroyed, or the Avatar Spirit is compromised by a spiritual infection.
As part spirit, the Avatar possesses an innate connection to the Spirit World and is at their strongest in that realm, due to being surrounded by spiritual energy. The Avatar used that connection to the Spirit World to be the bridge between the two worlds in order to keep peace between them and ensure harmony. After the Harmonic Convergence in 171 AG, however, Avatar Korra left the spirit portals open and renounced her role as bridge between the two worlds to let humans and spirits live together in harmony. The Avatar is also capable of recalling memories from past lives. This ability, however, can be injured or even completely severed. Avatar Aang unintentionally weakened his connection to all his past lives who came before Avatar Roku when he broke off his relationship with the Fire Nation Avatar, though he managed to restore the connection. When Raava was ripped out of Avatar Korra by Vaatu and subsequently destroyed, Korra's link to her past lives via Raava was completely severed despite the light spirit's revival.
Each nation has a method of recognizing the new Avatar. Among the Air Nomads, this involved presenting infants with a selection of toys and watching for a child that selected the exact four that had belonged to past Avatars, called the Avatar Relics. Traditionally, the child is told of his identity as the Avatar on his sixteenth birthday. However, Avatar Aang was told at age twelve, in fear of a war starting due to the Fire Nation's growing aggression, and Korra was discovered as the Avatar when she was just four years old: she exhibited bending abilities beyond her native waterbending without formal training.
The Avatar also possesses the ability to bend energy itself, although few Avatars have learned this technique, and even fewer have used it. Avatar Aang learned it from the last living lion turtle and used it to remove Phoenix King Ozai's Firebending, ending the Hundred Year War. He later used it to end Yakone's reign of terror in Republic City by stripping him of his Waterbending. Korra was also taught this ability after connecting with Aang when she lost her bending. Aang used this ability to restore her bending and she likewise did the same for Lin Beifong and many other victims of Amon's unique technique.
Upon mastering control of the Avatar State, after having learned to bend the four elements, an Avatar is referred to as "fully realized".