While not the largest group of animals they are certainly the most popular. Mammals are organisms that are multicellular, warm blooded, have hair/fur, and feed their offspring milk. They can be broken down into the monotremes (mammals that lay eggs), placental (mammals that have a placenta), and marsupials (mammals that carried offspring in a pouch).
Descendants of four-legged ancestors, reptiles are closely related to birds and amphibians. Unlike amphibians, reptiles have eggs surrounded by a protective membrane. We call this an amniotic egg. Most reptiles lay their eggs on lang. They have dry, scaly skin.
Amphibians lay eggs in water and most undergo metamorphosis. They are cold blooded. Amphibians can breathe through their skin, in addition to having lungs as adults and gills at juveniles. Approximately 90% of amphibian species are frogs.
Like mammals, birds are warm blooded. They lay eggs covered by a hard shell. Birds are the closest living relatives to dinosaurs. Birds have feathers.
Arthropods are invertebrates that have segmented bodies and jointed appendages. They can be broken down into insects, arachnids, myriapods, and crustaceans. The easy way to tell if something is an insect, crustacean, or arachnid is to count the legs. Insects have 6 legs - there are 6 letters in the word insect. Crustaceans have 10 legs - there are 10 letters in the word crustacean. Arachnids have 8 legs - there are 8 letters in the word arachnid. Myriapods have "many" legs as the prefix myria means many. Myriapods are centipedes and millipedes.
Cnidarians are aquatic organisms with radial symmetry. This means you can divide them in half many times, resulting in identical sections, like the spokes of a tire divide the tire into identical sections. Cnidarians are jellyfish, hydra, and sea anemones.
Echinoderms are marine animals. They consist of as sea stars, sea urchins, sand dollars, and sea cucumbers. They have radial symmetry. Most echinoderms can regenerate portions of their body. There are sea cucumbers that, when attacked, expel their intestines for attacker to eat. They then swim off and regenerate their internal components. Most reproduce asexually.