Korra is a 16-17ish year old girl from the Southern Water Tribe and the new Avatar (so yes, it means Aang is dead). Korra has mastered water, earth, and fire, but will go to the United Republic (a city founded by Aang and Zuko for people of all nations to live together) to seek out an airbender to teach her (if you want to know who, check out the link at the bottom).
Like how Ozai was the bad guy in A:TLA, Korra and her gang is up against The Equalists, a group of people skilled in chibending who live in the United Republic that are against benders and bending. They use their chi-blocking techniques (just like how Ty-Lee was able to do) to temporarily prevent benders to bend while in fights.
A very nice collection of info about the series so far (spoilers galore) is here.