Sugar is a term used to refer to any of a class of sweet, crystallizable, and water-soluble carbohydrates. When we refer to sugars we are mostly referring to the monosaccharides. and oligosaccharides, and sometimes is used specifically for sucrose, although this is not technically correct. Sugars are the primary source of energy for most animals and derivatives of sugars act as structural components of many molecules and substances in the human body (eg., glycosaminoglycans) and in plants (eg., cellulose).