the Legend of Korra has been on nickelodeon for a while and I like it. the characters are pretty good, though I wish they would have done more with the original Avatar and not left it at that cliff hanger where you find out Zuko's mother is still alive. If you want to find out more with that, you have to read the comics.
It stands for Legend of Korra [link], the spin-off/sequel to Avatar: The Last Airbender. They're animated, technically 'cartoons,' but don't let that fool you -- both are great, imaginative, and well-made series