Reptiles and amphibians are similar in many ways, and they are in fact distantly related, but there are several primary distinguishing features that clearly separate the two. There are a number of typical physical differences between the two types of animals, they often live in different environments, and they have different life stages. The animals' relationship with water is at the heart of many of these characteristics. At the most basic level, amphibians must spend at least some of their time in water, whereas reptiles do not have this requirement.
The Role of Water
It is true that many reptiles live in the vicinity of water, and some, such as alligators, strongly prefer a watery habitat, but this is not strictly essential for them to stay alive. Some, including certain lizards and snakes, are ideally suited to dry environments such as deserts, and rarely encounter moist areas. By contrast, many amphibians must remain moist in order to breathe; although they do have lungs, they also take in oxygen through their skin. This thin skin means that they lose moisture easily as well, and some will even die if their skin dries out completely.
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When reptile eggs hatch, the young look like miniature adults. As the young grow up, they will mature into scaly animals with fully developed lungs and dry skins.
An amphibian initially emerges from an egg in the form of an aquatic larva, like a tadpole, that breathes through gills. Tadpoles and other larvae cannot survive out of water, and as they grow and mature, they get bigger, grow limbs, and may lose their tails. Ultimately, most species develop lungs and turn into adults that spend much of their lives in and around the water. This means that amphibians will always spend the first part of their lives completely submerged in water, and essentially living like fish; indeed the word is Greek for "two lives," referring to dual staged life cycle that most amphibians go through.
Although both species are cold blooded, meaning that that their body temperatures fluctuate depending on their environment, the two are very different in appearance and physical makeup. Reptiles are covered in distinctive scales, and some may even have a rough texture. This is not true of amphibians, which typically have smooth, moist skin.
Reptiles also have more diverse body types than amphibians. They may or may not have limbs, and can range in size from very small to substantially larger and heavier than humans. Most adult amphibians have four limbs, and while their size varies, very few grow larger than a man’s arm. A rare exception, the Chinese giant salamander, occasionally grows as long as 6 feet (1.5 m.)
Eggs and Mating Habits
The vast majority of both types of animals lay eggs, but they are quite different in both quality and quantity. Amphibians lay large numbers — sometimes as many as hundreds or thousands at a time — of soft eggs that don't have an external membrane. They are usually attached to the stems of aquatic plants. The eggs of a reptile typically have hard leathery shells designed to protect the young inside from predators, and are often laid only a few dozen at a time at most in buried, insulated nests. It should be noted that a handful of reptile and amphibian species do not lay eggs, but have live birth.
Reptiles typically use internal mating mechanisms that are somewhat similar to those used by mammals. Amphibians usually mate externally, meaning that the female first lays her eggs in a large clutch, and the male then fertilizes them where they lay. Numerous rituals are specific to each individual species, but do not necessarily help distinguish a reptile from an amphibian.
Touch and Observation
Because of their major physical differences, handling or closely observing the animal in question is often enough to determine whether it is a reptile or an amphibian. Reptiles usually feel dry and scaly, and some species have very smooth skins while others have knobbles and horns. An amphibian will typically feel moist and sometimes rather sticky. Even from a photograph, the scales of a reptile should be clearly distinguishable, while amphibians have skin that usually looks slightly shiny, although depending on the species, the skin may be dull and without luster.