Avatar the Last Airbender is a tv show on Nickelodeon about a young boy who found out he was the Avatar, meaning he was the chosen one to bring peace and harmony between the four different nations. The four different nations existed because they each represented an element of which the different people that lived in them could bend. In the show, him and the friends he meets along the way go on a long adventure to defeat the one who has been destroying the peace, the Fire Lord.
The last airbender there was at this time and is the Avatar, also the main character.
She is Sokka's little sister, the last water bender in her tribe and the love interest.
Sokka is Katara's older brother, the only other one in the main crew that is a non-bender.
These three were also apart of the main crew and helped out in major ways.
Starting from the left, Toph is a blind, young female whos also the princess of a major earth tribe. She can bend rocks and at one point taught Aang how to bend earth.
Zuko is a fire bender and the prince of the entire fire nation that turned from being an enemy to a friend to help teach Aang how to bend fire. He is one the characters of the show that had one of the biggest character developments where he is trying to discover himself.
The last one is Suki and she is a non-bender but can fight extremely well. She is the leader of a female warrior group called the Kyoshi Warrior Team which helps defend Kyoshi Island where she lives. She is helpful to their little team because she is very reliable and her skills were definitely useful in fights.
Aang vs. Lord Ozai Final Battle