How does the body use sugar?

In simple terms, sugar is used by the body for immediate energy. Unused sugar leads to an increase in fat stores. All carbohydrates ultimately break down into sugars (which is why so many people fear carbs), but different types of carbs break down differently. Whole wheat has become so popular lately because it breaks down slower in the body than white flour, allowing for a more steady release of sugar into the blood stream to be used, without having the body be overloaded with sugar all at once, and storing the unused sugars as fat.

Carbs/sugars are necessary for humans to function optimally, but consuming them in excess inevitably leads to weight gain.