They have the ability to maintain their body temperature by internal means. The technical word for this is endothermy. Birds are the only other group of animals in the world that can do this. Feed their babies milk. This unique body covering is made of keratin and grows from special cells located in the skin.
Some mammals are hairier than others.
Marine mammals such as whales may be virtually hairless. Have a four-chambered heart. This is the most advanced kind of heart. Have live birth. In marsupials, such as the opossum, the young are born in a very primitive state and then finish their development in an external pouch.