Hentai is not just naked toons, has to be Anime. Its like squares compaired to rectangles, a square is a rectangle but a rectangle is not a sqaure.
Anime(as defined by dictionary.com)-A style of animation developed in Japan, characterized by stylized colorful art, futuristic settings, violence, and sex(thats not too acurate really, theres anime taking place in the past, has no violence, and is made for kids, anime is just the style.)
Hentai- same except the girls with the unusually large breasts are naked.(Commonly know for the tenticals)