And to follow up with the previous link, Matthew Bischoff with some apt advice for developers looking to differentiate themselves:
It’s not exactly an elevator pitch, or a mission or vision statement, and it’s definitely not a spec. It’s just a definition. It’s what you would see if you looked your app up in the dictionary. It answers the questions “What?” and “For whom?”.
There’s enough buyers on the app store that it’s possible to create an app that serves a very specific niche, and people will buy it.