There are more than 5,000 known species of mammals alive today. There are three major groups: monotremes (egg-laying mammals), marsupials (pouched mammals) and placentals (mammals whose babies develop inside the mother's womb, fed by a placenta). By far the largest group is the placentals - that's where human beings fit in.
There are several characteristics that all of us have in common: 1) we nurse our young from mammary glands, hence the name "mammals;" 2) we have hair or fur. Other things that tend to set us apart as a group include our adaptability and our relatively large brains.
Almost all mammal babies suckle milk from their mother's teats, though some babies nurse on hairs near the mammary glands. Young mammals mature more slowly than most other animals, and most usually "graduate" from milk to other food later in their development.
Fur or hair not only keeps mammals warm but also helps keep their body temperature constant (we're known as endothermic, or warm-blooded). There can be a great variety in some mammals' fur: from a few stray hairs on the chin of a whale to the dense fur coat of a chinchilla to the modified guard hairs known as "quills" on a porcupine.
Variety is the Spice of Mammals
As a group, mammals are very adaptable and occupy a wide range of ecological niches. Some live underwater (whales and dolphins) while others live high in the treetops (many monkeys). Some live in cold polar regions (polar bears and arctic foxes) while others live in hot deserts (kangaroo rats and fennec foxes).
Mammals have a wide range of body sizes. The common shrew weighs only three grams, while the blue whale weighs 150 tons! We also vary in the way we get around: from flying, swimming, and running to swinging through trees and burrowing.
Mammals have larger brains than other vertebrates of similar size, and seem to be the most capable learners. We also benefit from the relatively long amount of time it takes us to grow up, which gives our parents more time to teach the young important skills.