Simple, water soluble, carbohydrates, for example monosaccharides, disaccharides and oligosaccharides, may loosly be referred to as 'sugars'. They contain either an aldehyde of a ketose group and also contain -OH groups. By changing the orientation of the -OH groups around the carbon atoms can change the type of sugar. Sugars have the emperical formula CH2O and usually containg between 3 and 8 carbon atoms.
Sugar is a general term used for any mono and disaccharides. (For example dextrose which is a monosaccharide). It is a water soluble carbohydrate. The sugar glucose is converted during the process of glycolysis into pyruvate.
It is used as a store of energy within biological systems and can also be used as a sweetener and preservative.