Although i wouldn't refer to it as a substance, Carbohydrates are a nutrient that provides the energy for organisms. Simple carbohydrates like glucose (humans and animals) or Fructose (plants, specifically of the fruit), are used as easy energy by the organism. Complex carbohydrates are used to store large amounts of energy until the body needs it, and are usually large chains of glucose (in humans and animals) or fructose and sucrose (in plants). These chains form up Glycogen and Starch (humans/animals and plants, respectively).
Hope this helps, If you have any further questions on the topic feel free to comment.