Code: Lyoko

Code: Lyoko is about an evil artificial intelligance called, "X.A.N.A," it's a virus in the world of Lyoko, a virtual world. In the first season, Jeremie discovers Aelita, whom he believes to be a virtual being in the world she inhabits, but then later, in the second season, finds out that she was a human here on Earth. Ulrich, Yumi, and Odd are transferred to Lyoko and have to deactivate towers in the 4 sectors of Lyoko, Forest, Ice, Desert, and the Mountain sectors. Then they discover the heart of lyoko, where X.A.N.A lives, called, "Sector 5" or "Carthage." For fear of others shutting Lyoko down before they free Aelita from X.A.N.A, they keep this place a secret from the rest of the world.  

I believed the first season of this show sucked because, of the animation was quite bad and had many errors in the show. Though I didn't think the animation was very good, I liked how detailed the sectors were in this season. In the next season, you see a lot of changes in the characters and animation, there are fewer errors and the animation is a lot better After the second season, the animation doesn't change a lot until you reach the fourth season, where Lyoko gets a totally new look.

This show is in it's fourth season, which will be airing in America in Spring, it has already started in France I believe. 5 DVDs have been released so far, including, Vol 1, Vol 2, Vol 3, and the complete first season, along with vol  4 of the second season. They have planned 3 videogames so far.