Milk is the product of mammals. All female mammals make milk, and all mammal babies drink it -- that is the definition of mammal-ness!
Milk is an amazing beverage. Human children routinely live on nothing but mother's milk for six months to a year, and many go as long as two years. When you consider that a two-year-old kid can walk, talk, argue with you, etc., that's a pretty long period of time to go without ever touching solid food!
In order for milk to sustain a child for two years, it has to be a perfect food. That is, it has to provide the child with every single nutrient a child needs in exactly the right proportions. Each mammal's milk is tuned to the needs of its young so that it is perfect for the species. Milk can also provide all of the fluids a child needs. In environments where the water quality is poor, the mother's body acts as a filter so the child receives germ-free fluids.
Take human milk as an example. Human milk contains everything that a human child needs:
- Human milk contains 8.5% carbohydrates, mostly in the form of lactose. See How Food Works for a discussion of carbohydrates.
- Human milk contains 4.5% fat. See How Fats Work for a discussion of fat. Human milk also contains the essential fatty acids a child needs.
- Human milk contains 1.1% protein. See How Food Works for a discussion of protein. Human milk contains all of the different amino acids that a child needs in the right proportions.
- Human milk contains all of the vitamins and minerals the child needs. Minerals make up 0.2% of milk.
- Human milk also contains antibodies and other immune system components to protect the child (whose immune system is still developing) from disease.
- Human milk is 87.4% water.
The milks of different animals have different ratios. For example, dolphin milk is 14% fat and 10.4% protein. Whale and anteater milk are both more than 20% fat! This page has a nice chart of many different mammals.
It turns out that cow's milk is fairly close to human milk in terms of its percentages. Cow's milk has a little less fat, a little less sugar and about three times more protein than human milk does. It is not perfect like human milk is in terms of vitamins, minerals, antibodies, etc., but it certainly is a good all-around food for children to consume because it is so balanced.