Frogs are amphibians in the order Anura (meaning "tailless", from the Ancient Greek an-, without + oura, tail). The oldest fossil "proto-frog" appeared in the early Triassic of Madagascar, but molecular clock dating suggests their origins may extend further back to the Permian, 265 million years ago. Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is found in tropical rainforests. There are 4,800 recorded species, making them among the most diverse vertebrate groups. The body plan of an adult frog is generally characterized by a stout body, protruding eyes, cleft tongue, limbs folded underneath and the absence of a tail. Besides living in fresh water and on dry land, the adults of some species are adapted for living underground or in trees.

The skin of the frog is glandular, with secretions ranging from distasteful to toxic. Warty species of frog tend to be called toads. Frog warts (or verrucae) are elevations in the skin where glandular toxins tend to concentrate. The distinction between frogs and toads is based on informal naming conventions concentrating on the warts rather than taxonomy or evolutionary history; some toads are more closely related to frogs than other toads. Frogs' skins vary in colour from well-camouflaged dappled brown, grey and green to vivid patterns of bright red or yellow and black to advertise toxicity and warn off predators.

Frogs typically lay their eggs in water. The eggs hatch into aquatic larvae, called tadpoles, that have internal gills and tails. They have highly specialized rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets. The life cycle is completed when they metamorphose into adults. A few species deposit eggs on land or bypass the tadpole stage. Adult frogs generally have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frogs are extremely efficient at converting what they eat into biomass, which makes them an important food source for predators. Frogs are a keystone group in the food web dynamics of many of the world's ecosystems. The skin is semi-permeable, making frogs susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. Frogs produce a wide range of vocalizations, particularly in their breeding season, and exhibit many different kinds of complex behaviours to attract mates, to fend off predators and to generally survive.

There is a significant decline in amphibian populations affecting many regions. In 2006, one third of all frog and salamander species (33.6%) that depend on water during some stage of their life cycle were considered to be globally threatened or extinct. The number of malformations among frogs is on the rise and an emerging fungal disease, chytridiomycosis, has spread around the world. Conservation biologists are working to understand the causes of these problems and to resolve them.

Frogs are valued as food by humans and also have many cultural roles in literature, symbolism and religion.

Etymology and taxonomy
The name frog derives from Old English frogga, (compare Old Norse frauki, German Frosch, older Dutch spelling kikvorsch), cognate with Sanskrit plava (frog), probably deriving from Proto-Indo-European praw = "to jump". Approximately 88% of amphibian species are classified in the order Anura. These include around 4,810 species in 33 families, of which the Leptodactylidae (1,100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. Salientia (Latin salere (salio), "to jump") is a stem group including modern frogs in the order Anura and their close fossil relatives the "proto-frogs" (e.g., Triadobatrachus and Czatkobatrachus). The common features possessed by the "proto-frogs" nested within the Salientia group include fourteen presacral vertebrae (modern frogs have eight or nine), a long and forward sloping ilium in the pelvis, the presence of a frontal parietal bone and a lower jaw without teeth. The earliest frog fossil that falls into the anuran lineage proper, Prosalirus bitis, lived in the early Jurassic.

The use of the common names "frog" and "toad" has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins and the term "toad" generally refers to species that are terrestrial with dry, warty skins. There are numerous exceptions to this rule. The fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat, whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.

Anura includes all modern frogs and any fossil species that fit within the anuran definition. The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.

Frogs legs are eaten by humans in many parts of the world. Originally they were supplied locally but overexploitation led to a diminution in the supply and now there is a world trade in frogs. The main importing countries are France, Belgium, Luxembourg and the United States while the chief exporting nations are Indonesia and China. The annual global trade in the American bullfrog (Rana catesbeiana), mostly farmed in China, varies between 1.2 and 2.4 million tonnes.

Frogs are sometimes used for dissections in high school and university anatomy classes, often first being injected with coloured substances to enhance the contrast between the biological systems. This practice is declining with increasing concerns about animal welfare and "digital frogs" are now available for virtual dissection.

Frogs have served as experimental animals throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system through studying frogs. In 1852, H. F. Stannius used a frog's heart in a procedure called a Stannius ligature to demonstrate that the ventricle and atria beat independently of each other and at different rates. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy assays in the first half of the 20th century. A sample of urine from a pregnant woman injected into a female frog induces it to lay eggs, a discovery made by the English zoologist, Lancelot Hogben. This is due to the fact that a hormone, human chorionic gonadotropin, is present in substantial quantities in the urine of women during pregnancy. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer. This was the same technique later used to create Dolly the sheep and their experiment was the first time that successful nuclear transplantation had been accomplished in higher animals.

Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy assays have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable and can easily be kept in the laboratory. Xenopus laevis is increasingly being displaced by its smaller relative, Xenopus tropicalis, which reaches its reproductive age in five months rather than the one to two years taken by X. laevis, thus facilitating faster studies across generations. The genome of X. tropicalis is being sequenced.

Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller two hundred times more potent than morphine, is found in some species of poison dart frogs. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection. Dart poisons are under active investigation for their potential as therapeutic drugs.

It has long been suspected that pre-Columbian Mesoamericans used the venom of the cane toad (Bufo marinus) as a hallucinogen but research shows that it is more likely that they used substances discharged by the Colorado River toad (Bufo alvarius). This contains bufotenin (5-MeO-DMT), a psychoactive compound that has been used in modern times as a recreational drug. Typically, the skin secretions are dried and then smoked. Illicit drug use by licking the skin of a toad has been reported in the media but this may be an urban myth.

Exudations from the skin of the golden poison frog (Phyllobates terribilis) are traditionally used by native Colombians to poison the darts they use for hunting. The tip of the projectile is rubbed over the back of the frog and the dart is launched from a blowgun. The combination of the two alkaloid toxins batrachotoxin and homobatrachotoxin is so powerful that it has been estimated that one frog contains enough poison to kill about 22,000 mice. Two other species, the Kokoe poison dart frog (Phyllobates aurotaenia) and the black-legged dart frog (Phyllobates bicolor) are also used for this purpose but are less toxic and need to be heated over a fire to extract the poison.

Cultural beliefs
Frogs feature prominently in folklore, fairy tales and popular culture. They tend to be portrayed as benign, ugly and clumsy but with hidden talents. Examples include Michigan J. Frog, The Frog Prince and Kermit the Frog. The Warner Brothers cartoon One Froggy Evening features Michigan J. Frog who will only dance and sing for the demolition worker who opens his time capsule but will not perform in public. "The Frog Prince" is a fairy tale about a frog which turns into a handsome prince after he has rescued a princess' golden ball and she has taken him into her palace. Kermit the Frog, on the other hand, is a conscientious and disciplined character from The Muppet Show and Sesame Street; while openly friendly and greatly talented, he is often portrayed as cringing at the fanciful behavior of more flamboyant characters.

Toads have a more sinister reputation. It was believed in European folklore that they were associated with witches as their familiar spirits and had magical powers. The toxic secretion from their skin was used in brewing evil potions but was also put to use to create magical cures for human and livestock ailments. They were associated with the devil and in John Milton's "Paradise Lost", Satan was depicted as a toad pouring poison into Eve's ear.

The Moche people of ancient Peru worshipped animals and often depicted frogs in their art. In Panama, local legend held that good fortune would come to anyone who spotted a Panamanian golden frog and some believed that when one of these frogs died, it would turn into a golden talisman known as a huaca. Today, despite being extinct in the wild, Panamanian golden frogs remain an important cultural symbol and can be found on decorative cloth molas made by the Kuna people. The also appear as part of the inlaid design on a new overpass in Panama City, on T-shirts and even on lottery tickets.