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
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.