Vertebrata are classified into five groups based upon several characteristics.
Warm vs Cold blooded, mammals & birds vs fish, reptiles, & amphibians.
Mammals & birds can be differentiated by hair or fur vs feathers.
Mammals also have mammory glands so the females feed milk to their young.
Reptiles have dry scaly skin and lungs.
Amphibians have moist skin (not scales) and gills when young and lungs as adults.
Fish have gills throughtout life and live in water.