Maltose
Maltose is a disaccharide of 2 glucose sub-units. Maltose is a common component of "malt" produced from grain (eg., barley) starches.
Sucrose
Sucrose is a disaccharide of glucose and fructose. Sucrose ("table sugar") is commonly extracted from sugar cane or sugar beets. Sucrase is the digestive enzyme (produced by the intestinal mucosa) necessary to hydrolyze sucrose in order that the resulting free sub-units of fructose and glucose can be absorbed.
Lactose
Lactose is a disaccharide of glucose and galactose. Lactose is the "milk sugar" synthesized by the mammary gland and secreted in milk by lactating mammalian females. Lactase is the digestive enzyme (also produced by the intestinal mucosa) which is necessary to hydrolyze lactose in order that the resulting free sub-units of galactose and glucose can be absorbed. Lactase is an "inducible" enzyme: dietary sources of lactose will induce the production of lactase. Suckling animals have high levels of lactase to aid the digestion of lactose from milk. Lactase levels fall if the diet contains no lactose. "Lactose intolerance" occurs in individuals which cannot produce sufficient levels of lactase to aid the digestion of lactose. As a result, microbial fermentation of lactose occurs (and the associated production of "gas").