The quick answer: New Adult fiction focuses on characters in their late teens to early twenties, often involving a coming of age arc concerning careers, separation from their parents, and romance. Like Young Adult, it is engaging, quick-paced, and emotionally fulfilling, but it deals with slightly older viewpoint.
My own books focus on twenty-somethings figuring out what they’re going to do with their lives (because what can you do with a BA in English?) and if they can manage it without disappointing their parents, ending up in debt, or selling their souls. There’s also a heavy dose of romance and humor, along with good chocolate, football, and autumn in New York.
The, ahem, longer and somewhat dialectical answer can be found on my blog, where I interview myself about New Adult.