Vertebrates are so-named primarily for the definitive characteristic of having a backbone. Vertebrates also typically have multiple cells and a digestive cavity. There are seven types of vertebrate, including fish, amphibians, reptiles, birds and mammals. Invertebrates, on the other hand, are animals that have no spinal column, or backbone. Australia's science agency CSIRO estimates that 80% of all invertebrates are classified as Arthropoda, which is a group that includes spiders and insects.
Mammals are those creatures that conceive offspring in the reproductive tract of the mother and that are fed milk from the mother's mammary glands. Humans are mammals, as are whales. Mammals are warm-blooded, meaning they maintain a constant body temperature -- regardless of environmental factors -- due to their physical composition of sweat glands and insulating hairs. Mammals have strong jaws with specialized teeth which allow them to chew.
Amphibians can live on land and in water. Initially, amphibians begin life in the water, as they have gills. Eventually, they develop lungs which enable them to breath air while on land. Amphibians keep their skin moist by returning to water throughout their lives. Frogs and salamanders are examples of amphibians.
Reptiles include lizards, snakes, turtles, alligators and other scaled creatures. Reptiles have lungs to breathe air and often reside in the water, but they don't need to be wet to survive. Reptiles lay amniote eggs, meaning that their eggs contain amniotic sacs within a hard shell, which prevents them from drying out before hatching. Eggs that are laid on land are often safer from predators than those in the water.
Birds are like reptiles in that they lay amniote eggs as well. Unlike reptiles, however, birds are able to provide an even greater assurance of successful incubation by laying their eggs in areas that are out of the reach of predators, like treetops. Both birds and reptiles have eggs that are fertilized within the female's reproductive tract.
There are over 30,000 types of fish, most of which are cold-blooded vertebrates, exisiting in both fresh and salt waters. Fish primarily lay their eggs and can fertilize them either internally or externally. They constitute five sub-classes of the animal kingdom, which are all very different from each other.