According to the Penguin Dictionary of Biology (Abercrombie et al., 1966), Amphibia are a class of vertebrates represented by three orders: Anura (Frogs and Toads), Urodela (Newts and Salamanders) and Apoda (strange burrowing worm-like creatures called Caecelians). The defining feature of amphibians is that fertilisation is not accomplished by coition and eggs are unprotected by a shell and embryonic membranes.

Translation: What that means for non-experts is that amphibians (frogs, toads, newts and salamanders) do not have sex like us humans and other mammals. Generally eggs are laid in a big mass, outside the body, and fertilisation occurs by the male squirting his sperm over these eggs. This is known as external fertilization. Because of this method of reproduction, amphibians need water or moist soil for breeding. Without water, their eggs (which are not protected by a shell) would quickly dry out and the young would die before they even had a chance to develop. This is however, a generalisation, and there are many amphibians for which internal fertilisation is known. Similarly, there are a large number of amphibian species which require no water for fertilisation and have no free-swimming tadpole stage. They undergo direct development in burrows to adult frogs.

Thus amphibians derive their name from their need for water to breed and develop. The word is derived from the greek "amphibios" which means "double life", referring to the fact that frogs spend half their life near water breeding and developing, and the other half away from the water.