Mammals have several unique characteristics that differentiate them from other animals.
Most mammals have hair, or fur, covering their body.
They are also capable of regulating their body temperature. These allow the mammal to maintain a constant body temperature, regardless of the environmental temperature.
One other difference is that mammals give birth to young alive, and the female mammals produce milk to feed their young.
Most mammals walk on 4 legs, with only the humans walking upright on 2 legs. Aquatic mammals have flippers, or fins, for swimming rather than legs.