(Thar be some slight spoilers below for the second season (in the first paragraph) and the third season (in the one after).... >_>)

It's the story of a kid named Aang whose job as the Avatar is to master all four elements (water, earth, fire, and air) and go kick the Fire Lord's ass. He traveled with his friends Sokka, Katara (his waterbending teacher), Momo (a flying lemur), and Toph (his earthbending teacher who doesn't appear until the second season D;). They all travel around on Appa, Aang's flying bison. For the first season, the GAang's chased around by the angsty and banished Prince Zuko who believes capturing the Avatar will restore his honor and his father's love. Pffft By the second season, Zuko's changed a bit, having been labeled a traitor with his Uncle Iroh (who had been traveling with him for the first season too). This time, Zuko's younger sister Azula has taken to hunting down the Avatar when it's convenient with her two friends, Mai and Ty Lee.

The canon is ongoing, and is currently stalled with everyone left from the invasion team holed up in the Western Air Temple as well as Zuko (who's really changed for the better), Aang's firebending teacher.