There are about 4500 living species of class Mammalia, to which we belong. Mammals are homeotherms, with some species better at it than others. They have a four-chambered heart with complete double circulation; that is, the aerated blood of the arteries does not mix with the oxygen-depleted blood of the veins. The skin of most mammals is covered with hair at some stage of life. Mammals nourish their young with milk secretions produced in mammary glands of the mother. The fertilized egg develops inside the female; in most mammals, a special organ, the placenta, nourishes the developing embryo. Mammals have complex and differentiated teeth.
There are nearly 20 orders of mammals in two great subclasses: the subclass Prototheria, which includes the egglaying mammals of Australia, and the subclass Theria, which includes all other mammals. The duck-billed platypus and the spiny anteater, both prototherians, have a cloaca (a common channel for digestive, excretory, and reproductive products), a horny beak or bill (no true teeth), shelled and yolky eggs, a pouch, reptilian bones, and poor temperature regulation. Theria consists of two infraclasses Metatheria (the marsupials) and Eutheria (placental mammals). Most metatherians have an exterior pouch in which the young (born live) suckle for most of their development; they also have a cloaca and a double uterus and vagina. Eutherians have a single vagina; the young undergo considerable development inside the mother before they are born, and are nourished inside her by a special organ called the placenta. Eutherian orders include, among others, Insectivora (hedgehogs, shrews, and moles), Primates (lemurs, tarsiers, monkeys, apes, and human beings), Chiroptera (bats), Rodentia (squirrels, mice, and porcupines), Carnivora (dogs, cats, and bears), and Pinnipedia (seals and sea lions).
(From: Margulis & Schwartz, 1988)