The first season of "The Legend of Korra" was our introduction to the new avatar Korra, and our introduction to the world that's seventy or eighty years advanced from the one we saw in "Avatar: The Last Airbender." It was also about Korra learning to airbend, with the Season (or "Book" as they prefer to call it) titled "Air." This second season is called "Spirits" and is about the origins of the Avatar (not Korra specifically, but all Avatars), the way the spirit world interacts with the human world, and about Korra and Team Avatar learning to deal with those interactions.
Korra is young, impetuous, and occasionally hot-headed. This leads her to leave her air-bending mentor Tenzin in favour of her father's brother Unalaq who promises to teach her about the spirit world. And it's true, he's better qualified to do this than either her father or Tenzin. But his motives are suspect, and when she finally figures this out Korra says to Tenzin: "Everything Unalaq taught me was to help himself. Everything you've done was meant to help me." Which was true, and also a lovely lesson to the viewers to consider the motivations of the people who help you. This also shows the attitude of the show as a whole - combining fun characters and huge action set-pieces with relatively gentle moral lessons.
The original series had good artwork, But the artwork of this entire series is gloriously beautiful, just amazing - although blatantly CG in places. This season makes ideas barely hinted at in the first season ("the spirit world") into a massive, concrete thing that hugely impacts Korra and her friends.