Mammals are a class of vertebrate animal. If you are reading this, there is a very good chance you are a mammal. More specifically, you are a probably a primate of the human family.
The term mammal comes from the fact that female mammals have mammary glands that produce milk to feed their young offspring. Mammals are warm-blooded, air-breathing animals that have hair or fur on their body. Mammals also have three middle-ear bones to provide hearing.
There are three main classes of mammals: placentals, marsupials and monotremes. Most mammals give birth to live young using a placenta to carry the baby during gestation (i.e., placentals). Marsupials have a pouch in which females carry their young through early infancy. Monotremes, such as the platypus, are unusual mammals in that they lay eggs to give birth to their young.
There are approximately 5,400 species of mammals on Earth, divided into 153 families of animals. The largest family of mammals are the rodents, which include mice, rats, porcupines, beavers, capybaras, and other gnawing mammals. The next largest family are the chiroptera or bat family, followed closely by the soricomorpha family including animals such as shrews and moles. The next three large orders are the carnivores (such as dogs, cats, bears and weasels), cetartiodactyla (hoffed animals such as cows, deers, pigs and hippopotamus), and primates (such as humans, monkeys and apes).