According to Wikipedia, "Milk is a pale liquid produced by the mammary glands of mammals. It is the primary source of nutrition for infant mammals before they are able to digest other types of food."
Humans began to harvest milk from animals – typically cattle and goats – at the time of domestication of animals, between 9000-11000 years ago. This cultural change introduced a positive selection pressure for lactose tolerance, sparking a significant increase in the proportion of humans who could digest lactose. However, approximately 65% of the human population is still lactose-intolerant.
The production of animals' milk for human consumption is opposed by many for ethical reasons. Because mammals only lactate typically for less than a year after giving birth, the commercial harvesting of milk from an animal requires humans to keep the animals on a continuous cycle of artificial insemination, pregnancy and lactation periods. Artificial insemination is typically an unpleasant process for farmer and animal alike. In order to harvest the milk that calves would normally be consuming from their mothers, dairy cow farmers also separate each calf-mother pair, causing emotional pain and separation anxiety to the mother and the calf alike. Dairy cow mothers are known to "bellow" out in emotional pain for their calves for weeks after separation.
Cows can live for 25+ years in the wild, but dairy cows are slaughtered at 4 or 5 years of age, due to weakened ability to produce milk and physically deteriorated bodies from being kept continously on a strenuous pregnancy-lactation cycle. Their calves are slaughtered for veal if they are male, and are raised as dairy cows themselves if they are female.