A single sugar unit is a monosaccharide. Most nutritionally important sugars are pentoses or hexoses. Glucose, mannose and galactose are aldoses whereas fructose is a ketose.
Sugars link together via a glycosidic bond.
The nature of this bond influences the structural and chemical properties of the sugars and influences their ease of digestion. Sugars which bond via an alpha1,4 linkage may be digested by mammalian enzymes. Sugars which are linked via the beta1,4 linkage are resistant to digestion. This is digestible by salivary and pancreatic amylases. Nutritionally significant disaccharides are sucrose and lactose.