This is currently one of the best shows on Nick. Here is some information and a plot summary from the listing on Nick.com.
Water. Earth. Fire. Air. Only the Avatar was the master of all four elements. Only he could stop the ruthless Fire Nation from conquering the world. But when the world needed him most, he disappeared. Until now... On the South Pole, a lone Water Tribe village struggles to survive. It's here that a young Waterbender named Katara and her warrior brother Sokka rescue a strange boy named Aang from a cavernous iceberg. Not only is Aang an Airbender--a race of people no one has seen in a century--but they soon discover that Aang is also the long lost Avatar. Now it's up to Katara and Sokka to make sure Aang faces his destiny to save the tribe--and himself. Did we mention he's only 12? (TV Y7FV)
Here are some other pages about the show:
The show caught my attention due to the Earth/Air/Fire/Water thread in the plotline. Essentially, each element has it's own continent and nation. They lived in peace for centuries but then the Fire Nation attacked. The Airbender was a hero of old who was a master of all elements and was supposed to intervene in times of trouble and restore the balance. But when the hero was needed the most, he disappeared.
Now, many years later, a new Airbender has been found. He is powerful, but young and inexperienced. Will he become the hero he needs to be in time?
The elemental balance is marginally important in the spirtual lives of my favorite Dungeons & Dragons campaign setting, Dark Sun.