Fantasy
Oz is probably the ultimate land of fantasy, aside from Narnia and Middle Earth perhaps. Wicked has elements of fairy tales, magic, talking animals, lots of made-up places with odd names, and its very own fictional mythology.
But Wicked also has a lot of realistic elements as well, and deals with major political and social issues, albeit in a fantasy setting. In fact, these realistic traits put Wicked in the company of socially minded fantasy like Lord of the Rings, which has a ton of political allegory, and science fiction fare like Battlestar Galactica, which explored current issues like terrorism and governmental power. Fantasy stories are often ways to imaginatively explore contemporary social and political issues, and Maguire himself certainly approached Wicked with this in mind. In a 2008 interview he explained:
"[I]ndeed I hope it is fun. But once I get people involved in the plot, I hope to also communicate some of the questions I have about the way we live our lives in the 21st century." (source)