
In the world of Avatar there are different nations and each nation has an element, which the inhabitants can bend (control). These are:
The Fire Nation - Fire
Earth Kingdom - Earth
Air Nomads - Air
Southern, Northern Water and Foggy Swamp Tribes - Water
It is good to notice that as the events unfold each bending will have sub-skills which means that certain professional benders are able to do special techniques.
Certain notable sub-skills:
Fire: lighting generation, combustion, Earth: metal and lavabending, Water: bloodbending, plantbending, healing Air: doesn't have one yet
The Avatar is a person, who can bend all four elements and his role in the world is bringing balance and peace, which certain ppl tend to break.
Actually it is 2 show played in the same universe and these are:
The Legend of Aang
This is how everything has begun. In this show Aang, the last airbender in the world is the avatar. The main story is about that the Fire Nation wants to conquer the world and started a war against all of the nations. Certainly since the warmachines of the Fire Nation is quite advanced not many of the nations could survive the attacks. The main plot is stopping the Fire Lord. It has 3 seasons.
The Legend of Korra
After 50 years of the events of The Legend of Aang, Aang dies and a new avatar in the Southern Watertribe is born, who is named Korra. The world has undergone lots of changes. First technology has been improved a lot. A city has been founded for ppl from every nation, called Republic City. It doesn’t have a main plot and each season has different main story
Season 1: In Republic City the ppl, who can’t bend start a revolution against the benders. This group is called „The Equalists” and it is led by a person called Amon.
Season2: After the events of season 1 there will be a war between the Southern and the Northern water tribe. The chief of the Northern watertribe, called Unalaq wants to conquer the Southern Watertribe, because he has plans and he needs the Southern Watertribe for it.
Season3: After the events of season 2 certain ppl who couldn’t bend any kind of element gains the power of airbending and Korra wants to help to rebuild an almost dead nation, the Air Nomads. Meanwhile a group of 4 master benders have escaped their high-security prisons. Originally they wanted to kidnap the Avatar, but their plan failed and they were locked up in special prisons, which were designed to negate their abilities. And now that they have escaped they want to capture the Avatar.
Personally I had my doubts about it when I first heard the title, but when I started to watch it I liked it.

https://www.youtube.com/watch?v=YsZ267DRdq4
https://www.youtube.com/watch?v=9d4wovn-LdI