Reptiles and amphibians share some characteristics with mammals, but they are mostly different. For instance, all amphibians spend at least part of their life cycle in the water. Reptiles and amphibians also share many characteristics between them, such as laying eggs rather than giving birth to live young and being unable to regulate their body temperatures.
Both amphibians and reptiles are vertebrates like humans and other mammals, which means they have a backbone. Even turtles have backbones; theirs are part of their shells. Snakes, too, have a backbone. Reptiles were the first of vertebrate animals to live completely on land.
All reptiles have scales on their skin, but amphibians do not. Instead, most amphibians have a smooth, moist skin without any type of covering (scales, hair or feathers) that is used to help their breathing on land. If the skin of an amphibian dries out, a process called dessication), the amphibian will have trouble breathing.
All amphibians and most reptiles have three-chambered hearts. Two species of reptiles, the alligator and the crocodile, have a four-chambered heart like mammals do.
Most reptiles have claws on their toes. The exception to this are snakes and legless lizards, since they don't have legs. Amphibians do not have claws on their toes, however.
Both reptiles and amphibians lay eggs. Though amphibians go through a larval stage in their growth, reptiles do not. Reptiles lay their eggs on land, but amphibians can lay their eggs on land or in water. (Reptiles were the first animals with backbones to lay eggs on land.) Amphibians never give birth to live young, but some reptiles do.
Amphibians and reptiles are both examples of ectothermic animals, which means their body warmth is not produced and regulated internally by metabolism like humans and other mammals. This is why you will see snakes sunning themselves on rocks and why most larger reptiles and amphibians are found in desert and tropical climates.