Soluble, sweet, short-chained carbohydrates (molecules consisting of carbon, hydrogen and oxygen) like glucose, lactose, fructose, or galactose. Some consist of one unit (simple sugars or monosaccharides), others consist of two units (double sugars or disaccharides). Sugars can be freely available in food (e.g. fructose in fruit) or be joined by chemical bonds to form larger carbohydrates (like starch).
Glucose is a simple sugar unit that serves as the basic ""fuel"" for our energy metabolism. It can be ingested in the form of sucrose (a double sugar consisting of a unit of fructose and glucose) or as a part of larger carbohydrate molecules such as starch. These molecules are broken down by specific enzymes and the units of glucose are absorbed and transported around the body to where they are needed to supply energy.