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The typical features of mammals are: body (more or less) covered with hair; presence of the diaphragm muscle (that separates the thorax from the abdomen); mammary glands that produce milk (in females); enucleated blood red cells; middle ear with three ossicles.

The three groups into which mammals are divided are: monotremes (or prototherian, e.g., platypus), marsupials (or metatherian, for example, kangaroos) and placental (or eutherian, such as humans).

Mammals of the monotreme group (platypus, echidnas) are oviparous, egglaying, and they do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a placenta either; females of this group give birth to embryonic young that then continue development within the mother’s pouch. Placenta only forms in female placental mammals.

The orders into which placental mammals are divided are the following:

Artiodactyls, mammals with an even number of fingers in claws or paws like, e.g., cows, sheep, giraffes. Carnivorous, predators with canine teeth like dogs, lions, tigers. Cetaceans, aquatic animals without posterior limbs and similar to fishes, like whales and dolphins. Edentates, creatures with rare or absent teeth, like sloths, armadillos, anteaters. Lagomorphs, small-sized mammals having three pairs of continuously growing incisive teeth specialized in gnawing, like rabbits and hares. Perissodactyls, also known as ungulates (hooved), big-sized animals with an odd number of fingers in each paw, e.g., horses and rhinos. Primates, characterized by the big cranium and well-developed brain, like humans and apes. Proboscideans, big-sized animals whose nose and superior lip form the trunk (snout), e.g., elephants. Chiropterans, flying nocturnal mammals (bats). Rodents, animals with two pairs of continuously growing incisive teeth, e.g., mice, rats, castors, squirrels. Sirenians, aquatic mammals of freshwater, deprived of posterior limbs, like dugongs and manatees.

In mammals the embryonic development is direct, without larval stage. 

Mammals breathe through lungs, their respiration is pulmonary.

Cetaceans (whales, dolphins) and sirenians (dugongs, manatees) are aquatic mammals. Chiropterans (bats) are flying mammals.

Fecundation in mammals is internal, with copulation. In the contemporary world human technology is able to promote artificial external fecundation of human gametes and of gametes of other animals.

Mammals present a closed and complete circulatory system. The heart has four chambers and the arterial blood does not mix with venous blood.

Placental mammals reproduce sexually, they have internal fecundation and they are viviparous, i.e., their embryo develops within the mother’s body and from her it gets the nutrients through the placenta.

Like chondrichtian fishes and adult amphibians, mammals are ureotelic, i.e., they excrete urea.