Long ago, the four nations lived together in harmony. Then, everything changed when the Fire Nation attacked. Only the Avatar, master of all four elements, could stop them. Then, when the world needed him most, he vanished. A hundred years passed and my brother and I discovered the new Avatar, an Airbender named Anng. And although his Airbending skills are great, he has a lot to learn before he's ready to save anyone. But I believe, Anng can save the world.
“The strength of a woman is not measured by the impact that all her hardships in life have had on her; but the strength of a woman is measured by the extent of her refusal to allow those hardships to dictate her and who she becomes.”
Lion turtles are the creators of bending. There was one for each element. Wan, the first Avatar, asked each for the power of their element, in order to fight Vaatu, the dark spirit, with the help of Raava, the light spirit, in order to restore balance. In Avatar Aang's time, it seems that there is only one lion turtle left. It granted him insight into spirit bending, allowing him to become fully realized, defeat Fire Lord Ozai, and end the Hundred Year War.