Set 70 years after Avatar, it's about the new Avatar, Korra, a teenaged girl waterbender. A lot of the action is going to be taking place is a chaos-stricken, steampunk-filled metropolis called Republic City, and she's going to learn airbending from Aang's son, Tenzin.
The showrunners say it's going to be more "mature" than Avatar. Honestly, it sounds like it's going to be the Batman Beyond version of the show, and to me that's a good thing.