Larger brains, warm blood, and improved methods of caring for their young, have given mammals an edge in the competitive struggle for survival on earth.
Mammals are vertebrates that produce milk for their young.
Generally speaking most mammals have hair, although the whale which is a mammal, has none. Porcupines begin life with hair which turns into quills as they mature. One oddity of nature and an exception to our defintion of a mammal, is the Duck-Billed Platypus. The Platypus is a mammal which has hair, produces milk for its young, lays eggs, has a duck like snout and webbed feet. The fact that it lays eggs makes it an evolutionary mystery to scientists.