Carbs supply the body with glucose which is the preferred fuel for your brain/kidneys and red blood cells, carbohydrates are also the first fuel burned during exercise. There are two types of carbohydrates; sweet and starch. Starch including but not limited to; potatoes, corn and brown rice. Sweet including but not limited to; sugar, lollies and soft drinks.
The energy from starch based foods typically take longer to enter the blood stream and therefore provide a more sustained form of energy, often due to the fibre these foods contain.