Goat's Milk, Cow's Milk, Sheep's Milk

Milk is an emulsion AND a solution; you have fats, proteins, sugars, and various solids (mostly calcium) floating around in water. The proteins in Sheep and Goat are simpler and we digest them easier. Cow produces longer, more complex proteins and we, as humas, can have trouble digesting them. However, these same proteins breakdown differently during aging. This can result in the "sharp" flavor of aged cow's milk. Sheep's milk is usually the richest, with a greater variety and number of the various solids and sugars. Goats produce milk with less fat.  This is good.

Fun Fact: Ayrshire cows make milk with smaller fat globules. Jersey cows have a high amount of Carotene in the various solids department, which results in a yellower cheese.