In short, frogs are amphibians, living their lives both on land and in water. Amphibian require a water source to keep their skin moist. Because of their duel living situation, frogs possess both gills (to breathe underwater) and lungs (to breathe on land). Reptiles possess hard, scaly skin and only possess lungs. Frogs, toads, newts and salamanders are all examples of amphibians. Snakes, crocodiles, lizards, and turtles are examples of reptiles.
Frogs are not reptiles since reptiles do not undergo an aquatic larval stage as frogs do.
Reptiles are generally ectothermic (cold-blooded -- they regulate their body temperature via the environment), tetrapod (having four limbs, or descending from four-legged ancestors as snakes) animals. The class Reptilia is not universally recognized in scientific circles but is used by biologists and laypeople. Reptiles often reproduce via eggs, but some have live birth. Reptiles vary in size from small geckos to huge crocodilians. Reptiles are found on every continent except Antarctica.
Amphibians are also ectothermic tetrapods. The primary difference between amphibians and reptiles is in the mode of reproduction. Amphibians lay eggs in an aquatic environment (which might be nothing more than a catch basin in a tree). The eggs hatch into a larval form that has gills so that the animal can breathe in water; often the larvae can take oxygen in through the skin. Some adults retain this ability; indeed some adults do not have lungs at all. Frogs go through a tadpole phase living completely in the water before transforming to the adult stage.
The skin and habitats are what mainly set these two apart in classification. Amphibians have a smooth, moist skin and constantly rely on water to keep their skin moist. While reptiles have scale-covered skin. The snake's scales might be more dry than a frog's, but the snake's scales protect its skin from the sun and help trap moisture to keep the snake from drying up in the sun. This makes it not have to constantly rely on water like a frog. All reptiles use the same mechanism as snakes do (by trapping in moisture). All amphibians have to live in a moist, damp area to survive. This is the main difference between the two and explains why frogs aren't considered as reptiles. (This example is kind of like horses and camels)
I hope this answer helped.
A frog is not a reptile because it does not have the characteristics that set apart reptiles from amphibians. Reptiles have scales, lay eggs on land, and do not go through metamorphosis. A frog does not have scale, lays eggs in water and it goes through metamorphosis.
A frog is classified as an amphibian, not a reptile. We classify organisms into categories based on characteristics. All reptiles share the following characteristics: they are vertebrates (have a backbone); they are covered in scales; they breathe with lungs; most lay eggs (though there are a few exceptions that have live birth) and those eggs are laid on land; and most are cold-blooded. A frog is an amphibian because it lays its eggs in water and can breathes through its skin.
Frogs are amphibians, they live on both land and water. Frogs have moist skin. Reptiles, on the other hand, have a dry scaly skin.