Carbohydrate is currently the dominant food in our Western diet  Carbs include complex carbohydrates such as starch (found in vegetables, cereals, bread and pasta) and simple carbohydrates, such as sugar (found in sweet foods).
The carbohydrate element of any food is broken down by digestion into its smallest unit of glucose or sugar, and is transported around the body in the blood stream (hence blood glucose or blood sugar). The main role of carbohydrate, and why it is so significant, is to produce energy. It is your primary source of fuel, but not the only one.