He is an incarnation of the "Avatar", the spirit of light and peace manifested in human form.
As the Avatar, Aang controls all four elements (water, earth, fire, and air) and is tasked with keeping the Four Nations at peace.
Soon after, Aang meets a guru who attempts to teach Aang to open his seven chakras in order to control the defensive 'Avatar State'; but when Aang perceives Katara in danger, he leaves before the seventh chakra is opened, and thus loses his progress until the seventh is opened.
In the third and final season, Aang is unable to use the Avatar State.
After a series of misadventures, Aang meets his previous incarnation, Roku, who informs him that he must master all four bending arts and end the war before the coming of Sozin's Comet at the end of summer.
After helping the Water Tribe drive off a Fire Nation invasion headed by Admiral Zhao, with Katara as his teacher, Aang and his group journey to the Earth Kingdom to find an Earthbending teacher.
With this ability, as he regained his Avatar State, Aang removes Ozai's bending ability, rendering him harmless and ending the Hundred Year War.
The group learns about the Day of Black Sun in a secret underground library, and they attempt to reveal the information to the Earth King at Ba Sing Se.
However, their flying bison, Appa, is captured by Sandbenders.
Although reluctant with the plan at first, Aang accepts to have everyone think he had died and his remaining allies attack the Fire Nation's capital, but are thwarted by Azula.
During the finale, finding himself on a strange island, Aang is reluctant to actually kill Fire Lord Ozai, despite his four previous past lives (Roku, Kyoshi, Kuruk, and Yangchen) convincing him it is the only way.