These are the common types of animals that live on the planet Earth:

MAMMAL

Species in this group include humans, dogs, cats, bats, kangaroos, rhinos, hippos, apes, dolphins, whales. (From National Geographic.)
EndothermicWarm-bloodedBody regulates a near-constant temperature, enabling them to live in multiple regions or conditions.
ViviparousGive birth to live young.Only two mammal species hatch from eggs: the duck-billed platypus and the echidna (spiny anteater).
PelagePresence of hair or fur, even in small quantitiesprotection, heating and cooling, sensation
Endoskeletal VertebratesThey have bones that join to a centralized spine, including three bones in the middle ear.They're bones are the centralized support system INSIDE the skin.
Exocrine Glandssebaceous (fat secreting glands), sudoriferus (sweat), and scent glands.Most significant for mammals are the mammary glands.
Complex mental network nodeBrain is segmented into specific areas that control specific bodily functions, many autonomously (without thinking)The neocortex region creates and controls seeing and hearing
Heterodont dentationSpecialized teethIncisors, Cuspids, Bicuspids, Molars
Four-chambered heartTwo atria and two ventriclesKeeps "clean" (oxygenated) and "dirty" (carbonated) blood separate, while also helping to regulate body temperature.

AMPHIBIAN

Species in this group include frogs, toads, salamanders, newts, and caecilians (blindworms). (From National Geographic.)
Mostly Endoskeletal VertebratesThey have bones that join to a centralized spine.The legless caecilians have scales INSIDE their skin, which makes them classified as having an exoskeleton.
Ectothermic, or cold-bloodedThey cannot maintain their own body temperature and must have specific living conditions to survive.Too much sun, wind, or man-made toxins can damage their skin and eventually kill them. As a result, amphibians are the first to disappear from an ecology, and why over 50% of all frog species are endagered.
AquaticThey are usually born in water, and even when moving to land as an adult, they stay very close to their watery place of birth.Some species require high levels of moisture to survive instead of a body of water. It depends on the species.
Complex Cutaneous Glands1) some produce useful proteins, 2) some fight bacteria or fungus, 3) some are poisonous, as a defense mechanismThe most toxic amphibians are the most colorful.
HolometabolisticEgg-Larva-Adult Life Cycle Larvae are aquatic and free swimming, then reside mostly on land once their limbs and lungs have developed.
Carnivorous as adultsSome tadpoles and larval salamanders filter feed both plant and animal matterUsually consume invertebrates like insects, arachnids, and earthworms, but there are a few that are large enough to eat small vertebrates.
External fertilizationReproduce young outside the body in eggs
Cutaneous gas exchangeThey breathe through their skinWith primitive lungs (some salamanders don't have lungs at all), oxygen, carbon dioxide, and other minerals are passed through the skin using water.

AVES

Species in this group include ostriches, penguins, terns, cranes, hawks, macaws, and hummingbirds. (From National Geographic.)
Endoskeletal VertebratesThey have bones that join to a centralized spine.Although their wings are mostly cartilage, their skeleton is under their skin and feathers.
Pneumatized bonesBones are hollow for air intakeAllows for overall lighter weight (easier flight) and oxygen intake during flight
EndothermicWarm-blooded, regulate their own body temperatureCan live in a variety of climates and environments
External fertilizationYoung are produced outside the body in eggs, based on need for flightEggs are laid outside the body, in shells, instead of carried as extra weight.
Pregastric digestionBirds usually eat while they fly, and then digest later. Birds defecate regularly to keep their bodies light.
MurmurationThey have, or had at one time, adapted for flight (gliding and/or flapping)It is believed that ostriches, emus, and penguins could fly at one time.
Adaptive RadiationQuick evolution and adaption into multiple speciesMost scientists believe birds came from the therapod dinosaurs. There are more than 10,000 different species of birds alive today
MigratoryAround 20% of species migrate long distances every yearAlpine swifts can fly up to 6 months nonstop; the Arctic Tern has the longest migration in the world, flying 56,000 miles roundtrip every year from the Netherlands to Antarctica

FISH

Species in this group include goldfish, catfish, gar, eels, sharks, swordfish, clown fish, and salmon. (From National Geographic.)
Ectothermic VertebratesWith the exception of tuna and mackerel sharks, fish are cold-blooded, and cannot regulate their own body heat.They have bones that join to a centralized spine.
AquaticMust have water to remain oxygenated, and have gills instead of lungs.Most take in water through gills and separate the oxygen inside their bodies.Some fish with stationary gills must stay moving in order to keep the flow of water coming in (sharks). Others - like lungfish - come to the surface and gulp air.
Increased Olefactory and Auditory ResponsesWater transmits sounds, disperses chemicals, and conducts electricity better than air, so most fish don't rely too heavily on eyesight.Most fish rely on hearing, taste, and smell, and some can navigate and detect food with either motion or electrical sensors.
Extreme DiversitySince 70% of the planet is water, and each body of water is different, there are more than 32,000 different species of fish.There are more fish than the number of amphibians, reptiles, birds, and mammals combined.
Second only to insect class in number of species
The only animals with more numerous species than fish are insects.
Minimal AdaptationsVery old as an animal class.According to fossil records, fish have existed for more than 500 million years.
External FertilizationReproduction happens outside the bodyIn most cases, females release eggs in the water to be fertilized by the males.Baby fish are on their own from day one, and only the strongest and fastest survive to adulthood. It is theorized that fish instinctively travel in schools for protection, since they do not practice familial relationships.
Scales on top of skinA scale is a rigid plate that grows from the skin of a fish both protects it from outside threats, and helps keep a proper balance of water inside and pressure outside the body.There are cycloid scales (salmon), ctenoid scales (perch), ganoid scales (sturgeon), and placoid scales (sharks).Eating fish scales is not dangerous, but it's not pleasant tasting.
Fins for limbsInstead of 2 arms and 2 legs, and maybe a tail, fish often have 7 or more fins.The increased viscosity (thickness) of water allows for less gravity, and a greater range of motion.Water is closer to the conditions of the vacuum of space, which is why humans practice for space travel in water.
Swim BladdersInternal gas-filled organ that allows a fish to control buoyancyFilling and emptying the swim bladder, allowing the fish to move up and down, can keep a fish in motion for gill function without expending energy in the fins.The swim bladder is reportedly the inspiration for Greek scientist Archimedes' development of his theories of fluid dynamics.
BONUS FACT"Fish" is plural for "fish" when you have many of one type. "Fishes" is plural for "fish" when you have multiple species.When you have more than one salmon - you have many FISH.When you have salmon AND tuna AND sharks AND goldfish, you have many FISHES.

INSECT

Species in this group include praying mantis, locusts, dragonflies, butterflies, bees, and ladybugs. (From National Geographic.)
All insects are arthropods, with arachnids, lobsters, and millipedesInsects, however, have three pair of jointed legs, segmented bodies, antennae, and wings. They also are NOT vertebrates. Arthropods have exoskeletons - a bony shell protecting their insides.
Scientists who study insects, or bugs, are called entomologists.Entomologists believe that insects, the largest group of animals on the Earth, were also the first to leave the oceans for life on land.The oldest insect fossil was found in Scotland, and is dated to be over 400 million years old.
Some form of insect lives in every single habitat on the planet, from the oceans to the poles.So far, one million insect species have been identified, but four million are still uncategorized.A small fly called the Chironomid Midge is the only insect that can be found in Antarctica, usually during the summer. FUN FACT: Iceland is the only country without mosquitoes.
Insects are vital to every ecosystem, and provide a variety of services. They are pollinators, decomposers, and a food source.Birds alone eat eat between 400 to 500 million tons of insects per year.
Most insects reproduce oviparously, which means by the laying of eggs outside the female body.The eggs are normally laid near a food source, and the young pretty much have to survive on their own.In a few aphids and beetles, the insects are all female, and simply clone themselves to reproduce. This is called parthenogenesis.