Reptiles and mammals are animals that breathe air through the use of lungs. They are also vertebrates, which means they have backbones, and they are both descended from labyrinthodonts, large, extinct, freshwater amphibians with conical teeth.
Generally, mammals and reptiles are tetrapods, which means they have four limbs. The exception to this is the glass snake, which is a limbless lizard.
Mammals and reptiles have brains and a nervous system, and most members of both classes have eyes and ears. Snakes are an exception, as they lack external ears.
Because neither mammals nor reptiles need to live close to a water source to raise their young, both inhabit many types of environments. These include the oceans, the deserts and forests. Mammals' warm-bloodedness gives them a bit of an advantage in that they can live in the polar regions. Reptiles are generally not found in cold climates. One exception is the wood frog, which is found as far north as Alaska.
Like most mammals, some reptiles give birth to live young and nourish them until birth through an organ that resembles a placenta. Like mammals, some reptiles even care for their young for a time after they are born. Many reptiles and a few mammals lay eggs. These mammals include echidnas and platypuses.