Wikipedia: Milk

The composition of milk varies greatly among different mammals. Human milk, for example, is thin and high in lactose, its primary sugar. Cow's milk, in contrast, is lower in sugar and higher in fat. The milk of some mammals, particularly cows, goat?s, sheep?, and buffalo, is collected for human consumption, either directly or after processing into dairy products such as cheese.

Lactose in milk is digested with the help of the enzyme lactase produced by the bodies of infants. In humans, production of lactase falls off in adulthood, in many cases to the point where lactose becomes indigestible, leading to [lactose intolerance]?, a gastointestinal condition that afflicts many.

There is some controversy over whether consumption of cow's milk is good for adult humans, although it is generally recognized as beneficial for children. While milk is often touted for its significant amount of calcium, required for healthy bone growth and nerve function, there is some research to suggest that proteins in milk interfere with the use of its calcium to form bones.