Seeing as the two main characters are young (in the 12ish range), they make the typical mistakes that young people would—imagining themselves pitted against all of society, thinking that they're smarter than the adults around them, and so on. This makes them easy to empathize with, whether you're a kid or an adult, but we're thinking this one's particularly geared toward the younger set. So we're calling it Young Adult Literature.
Since the city of Ember is completely underground, its people are totally reliant on technology to live their lives. Artificial lights provide light to see by and grow plants with, while stores of processed food, vitamins, and medicine keep people healthy (sorta) without access to natural things like sunlight that we generally need to survive. But when the technology is starting to break down, it puts people's lives at risk.
As we find out, Ember was only built to accommodate human life for 200 or so years, so it's becoming an actively dangerous environment for the people still living there. In fact, the only reason they're living there in the first place is because some sort of terrible disaster has made the world above—our world—totally uninhabitable. And since their lives in Ember, post-nuclear-apocalypse, are pretty crummy, we're calling this one dystopian literature, too.