Water. Earth. Fire. Air.

The world of Avatar is all about the four classical elements. Some of its population are able to "bend" an element, magically controlling it. The world is divided into four nations: the Water Tribes (based on Inuit people), the Earth Kingdom (based on China), the Fire Nation (based on Japan), and the Air Nomads (based on Tibetan monks). In addition, there is a "spirit world", a realm layered on top of the real world, where all manner of weird and spiritual shit resides. There's weird shit in the normal world, too: most animals are a mixture of real world animals.

In order to maintain the balance between the four nations, there is the Avatar cycle: one person born to a nation that is able to not bend just that nation's element, but all four. When an Avatar dies, that power is reincarnated into someone born in the next nation in the cycle. The Avatar can learn to commune with the spirits of their previous incarnations for advice and knowledge. They can also, under extreme duress or training, access the Avatar State, borrowing the power of the past incarnations.