Carbs 101

Carbohydrates, like protein and fat, are a form of energy. In fact, they are the only form of energy used by the brain and central nervous system, and the body’s preferred source of energy, which is why most people eat a diet that is roughly 50% carbohydrates. Additionally, carbohydrates contain 4 calories per gram, are found in all foods that are not purely protein and/or fat, and come in a couple of different types: simple and whole grain. 
(Exercise.com)