Announced at the San Diego Comic-Con back in July 2010 as a spin-off (continuation) of Avatar: The Last Airbender, Avatar: The Legend of Korra is set for a 12 episode mini series premiering in November 2011. The series is set about 75 years(70-100 according to fan speculation) after the events at the end of the original series. Furthermore, the original series co-creators(Mike Dante DiMartino and Bryan Konietzko) have confirmed that they will personally write the 12 episodes, for a tighter storyline (much like how the original series began).
Some things to note so far, the new Avatar is a young woman from the Southern Water Tribe. Has already mastered three of the four elements, water, fire, earth, and seeks an Airbending master to complete her training. Most of the original series characters are gone, but Aang will likely make appearances in the spirit world, much the same way Avatar Roku did before him.
For more information and updates check out Avatar Wikia