Aang is an Air Nomad, and the current Avatar, the spiritual peacemaker of the world in human form. Thus being an Avatar, he is the only person who can master all 4 elements, Airbending, Waterbending, Earthbending, and Firebending. He is also one of the very few people who used Energybending, and the only one who used it in the series.
Before the start of the series, Aang, and his pet Flying Bison, Appa, had be frozen in an iceberg after a terrible storm for 100 years. When he came out, the world was in war, and he was still biologically 12. During his absense, the Fire Nation waged war with the other nations, and completely wiped out the Air Nomads. Now it's up to Aang to master the other 3 elements, defeat Fire Lord Ozai, and return balance to the world.