Tsukune failed his entrance exam. Luckily he was able to get into 1 school. He finds out that it is a school for youkai (monsters). On the way to his new school he meets a vampire girl named Moka. She's very self conscious from living in the human world through middle school. She quickly begins to cling to Tsukune. He also manages to attract a succubus named Kurumu, a snow woman named Mizore, and a little witch named Yukari. All 3 of these cute girls are all about him and fight like crazy people to win his affection. The problem he faces is his humanity. If he is discovered to be human he will be killed. The only other person that knows is Moka, but Moka is loving and loyal enough to keep his secret.
This season begins with all the characters going back for 2nd year. This time Tsukune meets Moka's sister, Kokoa, on his way to school. She's a feisty brat, and she's determined to remove Moka's seal and get her "real" sister back. There are also 2 episodes that involve Kurumu and Mizore's mothers. There just older versions of their daughters.
There is a descent amount of nudity in this season, unlike the 1st. So it's probably more of an R rating. There is not much of a difference as far as the animation and flow of the series is concerned. Thankfully, all of the voice actors have returned. I think they do a good job. I have seen rumors of a 3rd season, and I hope they are true. At the end of this season Moka tells Tsukune that he will be having to choose one of the girls very soon. That leads me to believe that a 3rd is likely. The popularity of this anime in Japan is also very large.
Overall this is a great harem anime with cool characters and lots of laughs. I really enjoyed it and I hope you do too!