In our world, we have people who are able to manipulate the elements, earth, fire, water and air. They're called benders. The Avatar is special in that they're able to bend all the elements instead of just one. I was just a firebender before I found out I was the Avatar.

Additionally, the Avatar reincarnation goes in a cycle base on the elements. Aang is an airbender, and Korra is a waterbender. After her, an earthbender would take her place and then it goes back to a firebender.