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Animal classes are groups that scientists put animals in when they consider them to be similar in some important ways.
The word amphibian is derived from a Greek word meaning double life, specifically a life that is lived on the land and in the water. All amphibians go through metamorphosis which a change from a larvae to and adult. Most often the larvae live in the water and the adults are able to venture out onto the land. Because amphibians skin needs to remain moist, they are most often found in or around water.
Birds are warm-blooded vertebrate animals that have bodies covered with feathers, are born out of a hard-shelled egg, have wings, no teeth, and a skeleton in which many bones are fused together or are absent. Birds that have the ability to fly have strong, hollow bones and powerful flight muscles.
Any animal that lacks a backbone is considered an invertebrate. They are usually soft-bodied animals that lack an internal skeleton for the attachment of muscles. Many invertebrates possess a harder outer skeleton (exoskeleton), that helps support the body and provide protection.
All mammals have these things in common: they have a backbone, they are born alive, they are warm blooded, they have body fur or hair, they have 3 middle ear bones, and they feed their young milk that females produce in special glands called mammary glands.
Reptiles are vertebrate animals that have scales covering their bodies, are cold-blooded, and reproduce by laying shelled eggs on land or by bearing their young alive.