If you haven't seen it, go do so now. Right now.
The Legend of Korra is the follow-up series. It's set many years after the adventures of the previous series, and most of those characters are deceased (Katara appears in a couple of episodes as an elderly woman), though their actions influence heavily this modern world. Time has moved on and now there are cars on the streets, electricity, radios and numerous other devices.
While Aang managed to settle the grievances between the various 'bending' nations ('benders' are people able to control one of numerous substances - earth, fire, water, air), there's now growing hostility between the benders and non-benders. Korra, the new Avatar (the only person who's able to control all four elements), arrives in Republic City initially to complete her air-bending training, but is soon involved in attempting to sooth relations between the citizens.
