Unlike other animals, mammals have body hair, three middle ear bones (the malleus, incus, and stapes), and nourish their young with milk that females produce in mammary glands.

Mammals are a group of warm blooded tetrapod vertebrates which are amniotes (having an embryonic sac or membrane surrounding the fetus). Mammals are distinguished from reptiles and birds by the possession of hair, mammary glands, a neocortex (a region of the brain) and three middle ear bones. The brain in mammals regulates body temperature and the circulatory system.

Mammals include the largest animals on the planet, the blue whale weighing in at 200 short tons (180 metric tons) and up to 108 feet (33 meters) in length. The most intelligent animals are also mammals, including elephants, primates and cetaceans (marine mammals).

The basic body type is a four-legged land-borne animal, but some mammals have been created for life at sea, in the air, or in trees.

The class of mammals has been divided into three subclasses and from 26 to 30 orders of which there is no consensus among biologists. The below list is the classes that include the vast majority of mammals. The below listed marsupial order is actually a subclass: (1) eutheria (placental mammals), (2) metatheria (marsupial mammals with pouches) and (3) prototheria (egg laying mammals).