Carbohydrates get their name from the elements that compose their molecular structure, namely carbon, hydrogen and oxygen.
Complex carbohydrates generally consist of long chains of glucose molecules, unlike simple carbohydrates (commonly referred to as sugars) which are formed from only one or two sugar molecules. Complex carbs can contain hundreds to thousands of molecules. That's quite a difference when compared to the single or dual molecule sugars. Single sugar molecules are called monosaccharide (mono- meaning one). All simple and complex carbohydrates are formed from single sugar molecules like glucose, fructose, and galactose joining together. These three single sugar molecules each have six carbon atoms, twelve hydrogen atoms, and six oxygen atoms, though their structures differ slightly.
Two sugar molecules come together to form simple carbohydrates called disaccharides (di- meaning two). Maltose (malt sugar), sucrose (found in honey and maple syrup), and lactose (found in milk) are all disaccharides, commonly known as sugars.
In contrast with simple carbohydrates, complex carbohydrates have several sugar molecules joined together, which is why they are also referred to as polysaccharides (poly- meaning many). Complex carbs can be categorized into three types; starch, glycogen, and fiber. Two common types of starches are amylose and amylopectin.
Starch and glycogen are easily broken down by our bodies, where fiber is not. Starches come from plant sources like grains, legumes, and tuber vegetables. White potatoes are known for their high starch content. Fiber, which can be either soluble (digestible) or insoluble (indigestible) is essential for coronary and intestinal health. Fiber is usually found in greatest quantity in the seeds, leaves, and stems of plants.
Glycogen is formed from glucose and stored in animals' liver and muscles, to be converted back to glucose when the animal needs it for energy. This also applies to humans. It cannot be found in significant quantities in plant foods. Because the human body also makes and stores glycogen, it is not necessary to get it from dietary sources.
Complex carbohydrates are often a more healthful source of energy than simple carbohydrates because of their slow breakdown. This means that our bodies get a steady supply of sugar over several hours instead of being bombarded with it all at once. It's easier for your body to process and use these slow-release carbohydrates, and prevents you from getting hungry as quickly.
This differs for athletes who use up their supply of glycogen and available glucose when doing marathon-type activities (high intensity or duration), in which case they would benefit from the immediate bioavailability of simple carbohydrates.