There are almost 5000 known species of frogs. They range in size from less than half an inch to nearly a foot long (two and a half feet with legs extended), and can be found in a fascinating variety of colors and patterns. In addition, new species of frogs are being discovered each year. However, even as these new species are being discovered, increasing numbers of previously existing frog species are going extinct. This is thought to be a consequence of the frog's extreme sensitivity to changes in their environment. For this reason, many scientists consider frogs an important "biological indicator" of the health (or lack of health) of the Earth's ecosystems.

Frogs are classified in the Kingdom: Animalia; Phylum: Chordata; Subphylum: Vertebrata; Class: Amphibia; Order: Anura (Salientia).

The golden poison frog (Phyllobates terribilis) is considered the most toxic animal in the world. A native of Columbian rainforests, its skin contains enough poison to kill 20,000 mice or 100 adult humans. Just two-tenths of a microgram of the batrachotoxin that it produces is lethal in the human blood stream. Traditionally, Embre and Choco Indians from Columbia have used this powerful poison to treat their blowgun darts. The darts are wiped over the back of the golden poison frog after heating it over a fire. Once a dart is poisoned, it will remain lethal for up to two years.

The largest known species of frog is the Goliath frog (Conraua goliath) which can grow to lengths of nearly 1 foot (head and body) or 2.5 feet with their legs stretched out. Living in fast moving rivers and streams in the rain forests of West Africa, the Goliath frog subsists on a diet of insects, crustaceans, fish, and other amphibians. Considered a food source by native tribes, an adult Goliath frog can weigh as much as a housecat (7 lbs)!

The African clawed frog (Xenopus laevis) was once used for pregnancy tests. A pregnant woman's urine has a very high concentration of the hormone hCG (human chorionic gonadotropin) which, it just so happens, will induce a female African clawed frog to lay hundreds of tiny transparent eggs within a matter of days. In order to conduct the test, a woman's urine was simply injected into the female frog, and if it began to lay eggs then she was pregnant.

The tomato frog is found only in Madagascar. It is considered endangered and, as a result, attempts are being made to breed it in captivity. As its name suggests, the tomato frog resembles a bright red, very ripe tomato.

The gastric-brooding frog (Rheobatrachus silus) is known for its unique reproductive process. Eggs are swallowed by the female and incubated in her stomach! Hormones produced by the developing young inhibit the normal digestive secretions of the mother's stomach and cause the upper intestine to temporarily shut down. Once they develop past the tadpole stage (about 6 to 7 weeks), the fully formed metamorphs (up to 25 of them) are regurgitated from their mother's mouth! The mother's digestive tract returns to normal after a few days and she is once again able to feed. Unfortunately, the gastric-brooding frog began to disappear not long after its discovery in 1972. It has not been sighted since 1981 and efforts to relocate the species have been unsuccessful. It is feared to be extinct.

A group of frogs is called an army. A group of toads is called a knot.

The oldest known frog fossil is a specimen of Triadobatrachus massinoti, a proto-frog that lived in Madagascar about 250 million years ago. A cross between a salamander and a frog, this 4 inch (10 cm) amphibian had a short body, a short tail and long jumping hind legs.

In 2003, evolutionary biologist Franky Bossuyt of Free University of Brussels in Belgium and colleague S.D. Biju of the Tropical Botanic Garden and Research Institute in Palode, India, discovered a new species of frog in the Western Ghats Mountains of Southern India. Genetic analysis soon indicated that this frog was like no other frog alive today. In fact, researchers soon announced that this short-limbed, 3-inch long creature often described as looking like a "jelly donut" belongs to a family of frogs thought to have disappeared millions of years ago! This new species, which has since been dubbed the purple frog (Nasikabatrachus sahyadrensis), may have evaded scientists all these years because it spends most of its time underground and only surfaces for about two weeks each year to mate. The purple frog is so unique that it has prompted the creation of a new frog "family".