Early fanart for Legend of Korra (next series from The Last Airbender universe). via mudron @flickr

From the Nickelodeon Press Release to WSJ.com

The Legend of Korra takes place 70 years after the events of ‘Avatar: The Last Airbender’ and follows the adventures of the Avatar after Aang – a passionate, rebellious, and fearless teenaged girl from the Southern Water Tribe named Korra.”

Korra’s quest eventually leads her to Republic City–the epicenter of the world of “Avatar.” A metropolis powered by steampunk-type technology, the city is inhabited by people from all nations. Korra finds that Republic City suffers from rampant crime and is also dealing with an anti-bender revolt. Korra is tutored by Aang’s son, Tenzin, is the ways of airbending.

Update #1: Further details

- the story takes place 70 years after The Last Airbender storyline

- Korra, the Avatar, already mastered 3 of the elements, and needs to learn Airbending