Katara: Why didn't you tell us you were the Avatar?
Also known as Avatar: The Legend of Aang in some countries, Avatar takes place in a Constructed World divided into four nations: the Water Tribes, the Earth Kingdom, the Fire Nation, and the Air Nomads. Each nation has developed a spiritual art form to manipulate ("bend") their namesake element, but only the Avatar -- one person reincarnated into each race in turn -- is capable of mastering all four elements. The Avatar's role is to serve as a peacekeeper and protector for all four nations (as well as bridging and balancing the physical and spiritual worlds).
The show's story centers around Aang, a twelve-year old (chronologically 112) who is the most recent incarnation of the Avatar and is the eponymous "last airbender", and his quest is to master the four bending arts so that he can save the world from Fire Lord Ozai and end the century-long war that he spent a hundred years trapped in an iceberg accidentally avoiding. Ozai's plans for domination are on a specific timetable, though, meaning that Aang has until the end of summer to stop them.