One of the best animated television shows on right now. Right up there with Timm & Dini's DCAU work. And unlike 'The Simpsons', 'Family Guy', etc...it's not a comedy and it is a serial. If you've never seen it take a chance and catch the ep tomorrow; you won't know what's going on precisely, but maybe you'll be intrigued enough to seek out Book 1: Water and Book 2: Earth to catch up.

Background info:
Avatar: The Last Airbender