Though all mammals have mammary glands and produce milk, not all mammals have teats. The exceptions are the two monotremes the Echidna and the Platypus. In monotremes the milk is secreted onto the surface of the skin like sweat and licked off the body hairs by the young. In later mammals the mammary glands became more complicated with the possession of special ducts which channel the milk into the teats. The number of teats varies from 2 in Primates and the Marsupial mole, to 19 in the Pale Bellied Oppossum.
Milk contains proteins, fats, carbohydrates, minerals and water.
The proportion of the various constituents is highly variable between different species of mammals. 6.23 Milk doesn't just leak out whenever, it generally requires a chemical and physical stimulus. The chemical stimulus to release milk is the hormone oxytocin produced in the pituitary gland. The physical release is normally the act of suckling. Milk also contains antibacterial and antifungal agents.