Essentially, its about four nations of people: the Air Nation, Water Nation, Earth Nation and Fire Nation. There's one person called the Avatar who is like a fictional version of the Dalai Lama (reincarnated into each Nation). He's supposed to keep balance between the Nations. Anyway, the cool part is the bending. Each Nation has people who have the gift of bending their specific element. The show incorporates martial arts movements which makes it even cooler in the sense that the people have to practice and train before they're any good at bending. Long story short, there are three seasons to the show, but a live action movie directed by M. Night Shyamalan is currently in production right now and will be in theaters July 2nd. Here's the link to the trailer.
I can't wait!