Majority of the people have an innie, or a concave belly button. Only about 10% of the population have an outie, a belly button that protrudes. The exact determining factor for an innie or outie belly button is unknown, and seems to be random.
Surveys conducted by various magazines have concluded that innies are universally seen as more aesthetically appealing than outies. People who are dissatisfied with their outies are going for umbilicoplasty, a belly button plastic surgery to convert an outie to an innie.