Lactose is the sugar making up around 2-8% of the solids in milk. The name comes from the Latin for milk, plus the -ose ending used to name sugars. Lactose is a disaccharide consisting of two subunits, a galactose and a glucose linked together. In the young of mammals, an enzyme called lactase is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits for absorption.
Normally, as the young grow up, production of lactase gradually ceases, and they are then unable to metabolise lactose. This is perhaps an evolutionary mechanism to enforce weaning of the young. This loss of lactase on maturation is also the default pattern in most of the human race. However, in some humans (mostly those with ancestry in the approximate geographic region of Europe, the Middle East and India), the enzyme is retained in adulthood, and dairy products form a substantial part of the adult diet. It would appear that millennia of animal husbandry in those regions, with the milking of sheep, cattle, goats and water buffalo, has caused an evolutionary adaptation to an adult diet containing milk. (This process of retaining infant characteristics into adulthood is one of the simplest routes of evolutionary adaptation, and is known as neoteny) The fact that at least some humans have made adaptations to lactose in the adult diet would, incidentally, appear to cast doubt on some arguments by proponents of the so-called `Stone-age diet', who argue that human metabolic needs have not changed since the last Ice Age.