Sit-ups, crunches and other exercises can strengthen your belly muscles, but there is no such thing as spot reduction. Exercising a muscle does not get rid of fat over the specific muscles that are exercised. If it did, tennis players would have less fat in their tennis arms, but they don't.
When you take in more calories than your body burns, you store them as fat. You store more that half the fat in your body underneath your skin and over your muscles. Some people store fat primarily in their hips and are at low risk for heart attacks and diabetes, while others who store their fat primarily in their bellies are at increased risk for heart attacks and diabetes. The "ab" exercises can strengthen sagging belly muscles, but they will not remove extra fat from your belly. The only way to lose fat from the place where you store most of your fat (whether it's your belly or your hips) is to lose weight overall.