Carbohydrates are the main energy source for humans, and are used for energy storage. They exist in different forms: monosaccharides (single molecules), disaccharides (two molecules together) or polysaccharides (long chain of molecules), each form contributing something different to the human metabolism.
Sugars, starches, and fibers are all carbohydrates.
Sugars are found in the mono- and disaccharide form. Glucose, fructose, sucrose, and lactose are all examples of sugars. Sugars are easily broken down in the GI tract and provide quick energy.
Starches are long chains of glucose molecules (a monosaccharide). They are the storage form of energy in plants and are also easily broken down in the human GI tract. Glycogen is a type of starch that is produced by the human metabolism and is found in the liver; it is used as a short-term storage form of glucose for quick energy.
Fiber is is the structural portion of plants and the most complex form of carbohydrate. It cannot be broken down in the human GI tract. There are two types: soluble and insoluble. Soluble fiber dissolves in water and ferments in the colon; it slows down digestion. Insoluble fiber does not dissolve in water, provides bulk to the stool and speeds up digestion.