Short answers, pulled from the story.
Avatar Aang's spirit reincarnates into a new body born in the next nation according to a fixed cycle that moves from Fire to Air to Water to Earth before repeating itself. When an Avatar dies, their spirit enters this cycle and must travel the world to learn all four bending arts before assuming their role as peacekeeper.
The series premiered on the 21st of February 2005 after a teaser reel appeared at Comic-Con in July 2004. It became the highest-rated animated television series in its demographic upon premiere with an average of 1.1 million viewers per new episode.
Michael Dante DiMartino and Bryan Konietzko met at a Halloween party in 1995 while studying at the Rhode Island School of Design and began their professional partnership that same year. They moved to Los Angeles in 1996 and 1998 respectively to pursue careers in animation before formulating the concept for the series during a stint as art director on Invader Zim.
Waterbending uses movements influenced by tai chi focusing on alignment body structure breath and visualization. Hung Gar inspired earthbending practitioners due to its firmly rooted stances and powerful strikes representing the solidity of earth. Northern Shaolin represents firebending through strong arm and leg movements while Bagua uses dynamic circular motions for airbending.
Aang traveled with Katara and Sokka to the Northern Water Tribe to learn waterbending after being revived from the frozen tomb where he had been encased inside an iceberg near the South Pole. He later confronted Fire Lord Ozai when Sozin's comet returned and stripped him of his firebending ability without taking a life.