Aang: is the Avatar, the only person who can master all four elements and bring balance to the world. He has the ability to control Air.
Katara: Katara is a Waterbender, meaning she can control water with her mind.
Sokka: Sokka is a non-bender, but he is a skilled warrior and strategist.
Toph Beifong: Toph is a Earthbender, meaning she can control earth and rocks. She also has seismic sense, which allows her to "see" by feeling vibrations in the ground.