From the introduction:
“Water. Earth. Fire. Air. 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, but when the world needed him most, he vanished. A hundred years passed and my brother Sokka and I discovered the new Avatar, an airbender named Aang. Although his airbending skills were great, he had a lot to learn before he was ready to save anyone. But from the first day I met him, I believed Aang would save the world. And you know what? I was right. With the help of his friends, Aang defeated Fire Lord Ozai and ended the Hundred Year War. Zuko, Ozai's son and our ally, became the new Fire Lord. Together with Earth King Kuei, Aang and Zuko promised to return the nations to harmony."
- — Katara