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Class Amphibia refers to Amphibian, which means "double life" or life on land, and in water. It includes the three modern orders, Anura (frogs and toads), Caudata (salamanders), and Gymnophiona (caecilians). All amphibians are vertebrae, and they all have thin skin permeable to air and water. Amphibians are cold blooded, which means they can't regulate their own body temperature. Amphibians breathe through both their lungs and gills, which makes them very unique.
Amphibians are typically in warm, moist, places.
http://www.shsu.edu/~bio_mlt/AMPHIBIA.html

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Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Caudata
Family: Salamadroidea
Genus Species: Caudata

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Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Family: Bufonidae
Genus Species: Bufo Marinus

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Kingdom: Animalia
Phylum: Chordata
Sub-phylum: Vertebrata
Class: Amphibia
Order: Anura
Family:  Ranidae
Genus: Lithobates
Species: Catesbianus

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Amphibians are in decline worldwide, and they're getting hit hard. So, some frogs in Africa are leaving their original habitats and venturing into forest areas, which is uncommon with amphibians, but they had to move to survive. The frogs that just found that habitat started mating with some of the frogs that already lived there, which cause a cross-breeding and development of new traits.

https://link.springer.com/article/10.1007/s10592-006-9272-0

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An example of natural selection in frogs is the gray and green tree frogs, and how when the trees change or their habitats have to change, the bright green tree frogs are more visible to predators than the more subtly tones of the gray frogs. If the green frogs are easier to see/eat, soon enough only gray frogs will live in that area, because they're the only ones who could survive.

https://spark.adobe.com/page/hMgxO/

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Amphibians were a part of divergent evolution, meaning 2 or more species evolved from a common ancestor. Amphibians, reptiles, and many more organisms evolved from tetrapods, an early vertebrae from which all vertebrae are thought to have originated from.

http://www.biology-online.org/dictionary/Divergent_evolution

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In some Amazonian frogs, there has recently been found proof of divergent sexual selection speciation. Because of this, strong behavioral isolation began to occur. Some males start becoming more interested in mating with neighboring populations, and the females only payed attention to the calls of the frogs from the other populations as well.  Some scientists argue that sexual selection is the primary driver of speciation.

http://rspb.royalsocietypublishing.org/content/274/1608/399

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Ichthyostega was a carnivore, differing in size from 0.5 - 1.2 m. The earliest Ichthyostega comes from 363 million year old deposits in Greenland. It was largely aquatic but had massive ribs that could have been used for support of internal organs while on land.

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Amphibia/fossilrecord.html
(picture b)

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Order Diadectomorpha. A Late Carboniferous to Early Permian group that were close to amniotes. Most were herbivores, although some were carnivores. Diadectes, from the Western USA, Was extremely heavily built with massive limb girdles, but with very short limbs.

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Amphibia/fossilrecord.html

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This is a more recent fossil, dating back to around 45-55 million years ago. This is a frog, as you can kind of see from the bone structure.

http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibfr.html

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Shown here is the Phlegethontia which is an early caecilian. You can see it's snake-like features, and how it has no legs but it still has vertebrae. It's extremely rare for caecilians to be fossilized, so when they are, we try to preserve them as much as possible. They're thought to be rare to find because of their burrowing habits and reduced skeleton.

Legless caecilians were found dating back to the Jurassic period, but before that, their ancestors had very short legs.

http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibfr.html

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Acanthostega was discovered in the 1990s and represent tetrapod that has hands and feet for which we have a full skeleton. It had toes, eight per limb, no fin rays, a large pelvis and is thought to have formed gills in adulthood.

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Amphibia/fossilrecord.html

(picture a)

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The Tiktaalik roseae fossils were recently discovered, and proved to be a direct link between fish and amphibians. They lived in shallow water and when they ventured on land, they had access to many new nutrients and few predators, which allowed for even more evolution to occur. This time period was later known as "The Age of Amphibians".

https://allyouneedisbiology.wordpress.com/2015/05/07/evolution-amphibians/

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Amphibians evolved from tetrapods, but more specifically, sarcopterygians which were a type of fish with a special bone in their fins which allowed legs to evolve from it.
It took a long time for amphibians to develop lungs, but they appeared from the gas bladder in the fish which controlled their buoyancy.
Amphibians then developed internal nostrils, which helped them breathe out of the water and be able to smell.

https://allyouneedisbiology.wordpress.com/2015/05/07/evolution-amphibians/

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Darwin's other theory was that everything descended from a common ancestor. All amphibians were thought to be descendants of tetrapods which were organisms that were vertebrae with forelimbs and hind limbs and ears. They were a superclass of animals that were the ancestors of every vertebrae, like mammals, birds, reptiles, and amphibians, which loosely relates them all.

https://www.britannica.com/animal/tetrapod-animal

http://www.sparknotes.com/biology/evolution/darwin/summary.html

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Order Nectridea. A group of aquatic early amphibians from the Late Carboniferous to the Early Permian. They were newt-like in looks, with long flat tails used for swimming, such as Sauropleura. Diplocaulus and Diploceraspis from the Late Carboniferous and Early Permian of the USA have expanded skulls with large horns growing out of the sides. The younger forms have no horns at all and they enlarge throughout life. The function of the horns is still unknown.

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Amphibia/fossilrecord.html

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In frogs, there is much evidence of natural selection. Natural selection is survival of the fittest; the organisms that are better suited for the environment will survive and create babies that are also better suited for the environment until that's all that there is. For example, a lot of tree frogs are a bright green which helps them blend into the leaves. Some tree frogs are more grey, and blend in better with the bark on trees. So in fall, when it's warm enough for the frogs to still be out, but the leaves are falling off of the trees, the grey frogs survive a lot better and they already lay more eggs, so eventually the green frogs won't live in that area anymore.

https://spark.adobe.com/page/hMgxO/

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Caecilians are some of the lesser known amphibians. They're legless, and the adults loosely resemble earthworms, except for the fact that they have jaws and teeth.

http://www.encyclopedia.com/plants-and-animals/animals/vertebrate-zoology/caecilian

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Frogs and toads are very similar but very different. Although they both spend some of their life in water and on land, frogs spend the majority of their life in water, while toads spend the majority on land. Frogs have small teeth while toads have none at all. Frogs have smooth, slick skin, but toads have dry, bumpy skin.

Baby frogs and toads are called tadpoles, and they live in the water, like fish. The frog eggs are laid in big clusters with a jelly-like substance around them. Toads, however, are laid in lines or strands on the leaves of plants in the water.

http://www.frog-life-cycle.com/frog-toad-difference.html

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Salamanders are different from other amphibians because it has a tail that stays attached during adulthood. The tails are taller than they are wide, and some can even hold on to branches-- like possums.

Salamanders' respiratory organs differ among the different types. Some salamanders have internal gills, some have external gills, and some breathe through their skin--- they absorb oxygen through their skin or the membrane on their throats or mouths.

Though most salamanders have four short legs (two in front, two in back) that stick out from the sides on their bodies, at least two species have only two front legs.

http://sciencing.com/salamander-characteristics-7873616.html