There are three main amphibian classes: frogs, caecilians and salamanders. They all share common characteristics. Amphibians can be distinguished from other animals by means of several characteristics, such that:
- they are vertebrates, since they possess a backbone;
- they cannot control their body temperature since they are cold blooded;
- amphibians could spend their lives on land or water;
- since amphibians breathe through skin, the skin structure has no scales, but it is permeable;
- amphibians go through metamorphosis.
Unlike terrestrial organisms, such as reptiles, amphibians produce eggs that have a different structure, meaning that the calcified shell common to most eggs is missing. The amphibian's eggs have a jelly structure, prone to dehydration if it exposed to air.
Amphibians are the most exposed to risks class of animals. They are susceptible to environmental threats, such as pollution or global warming.