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.
WATER...EARTH...FIRE...AIR... Long ago, monsters and humans lived together in harmony. Then, everything changed when the humans attacked. Only a human, who's the master of all four elements can restore peace and balance to the world.
“Republic City is the most advanced part of the Avatar world, while Republic city has advanced very far in the 70 years since Aang, the rest of the world is more similar to what it was 70 years ago.” Submitted by this person Art