Long ago the four nations lived in Harmony.Aang, a 12-year-old Airbender of the Air Nomads' Southern Air Temple, learned from the elder monks that he was the Avatar. Usually, the Avatar is told of his or her true identity after turning sixteen; however, the monks feared that a war between the four nations was on the horizon and that soon the Avatar would be required to maintain balance and order in the world. While traveling over frigid southern ocean waters, a sudden storm caused Appa to plunge deep into the sea. Aang unconsciously entered the Avatar State, and used a combination of Airbending and Waterbending to protect Appa and himself. By creating an air bubble around them and then freezing it, Aang was able to ensure that he and Appa could breathe and be protected from the storm until it dissipated. However, the storm transferred the bubble very near the south pole and it could not thaw, forcing the two of them into a state of suspended animation.Two teenage siblings from the Southern Water Tribe — Katara, an inexperienced Waterbender, and her brother, Sokka, a warrior and caretaker of the tribe — discover and free Aang and Appa from the iceberg. Aang soon discovers to his horror that, during his absence, a century-long war has been waged.
(taken and edited from wikipedia)

Picture	Real Name	Description
	Aang	Aang is the last airbender and the currrent Avatar.As the Avatar, Aang must master all four elements to bring peace to the world and restore the balance between the four Nations.
	Katara	Katara is 14 years old, and the sole remaining Waterbender of the Southern Water Tribe. Katara, along with her brother, Sokka, discovers Aang at the beginning of the series. She and Sokka accompany him on his quest to defeat the Fire Lord and bring peace to the war-torn nations.
	Sokka	Sokka is a 15-year-old warrior of the Southern Water Tribe who, with his sister Katara, accompanies Aang on his quest to defeat the Fire Lord. Sokka describes himself as "meat-loving" and "sarcastic."Unlike his companions, Sokka cannot Bend an element.
	Toph Bei Fong	Toph Bei Fong is a 12-year-old, blind Earthbender who leaves her wealthy lifestyle and home to join Aang on his quest in order to teach him earthbending. Toph has largely lived alone all her life due to overprotective parents, which makes her somewhat selfish, sarcastic, and, at times, bitter and arrogant. She also does not hesitate to speak her mind and is bluntly truthful. Though blind, Toph "sees" with Earthbending, feeling vibrations in the ground that objects make when they are moving.
	Zuko	The exiled 16-year-old prince of the Fire Nation. Zuko is obsessed with capturing the Avatar in his quest to restore his honor and redeem himself in the eyes of his father, Fire Lord Ozai.