Mammals are a class of species that regulate their body temperature internally. Except for monotremes, the young are born live and suckle milk from the female. Most have some hair or fur on their body.
Reptiles are of the class Reptilia. They lay eggs and their skin is covered with scales. They are often referred to as "cold-blooded" because unlike "warm blooded" mammals they cannot regulate their own body temperature internally. Instead their body temperature is determined by the ambient temperature of their environment.
Arthropods include insects, spiders, scorpions, and crabs. They are invertebrates that is they have no backbone, but have a exo-skeleton that protects them. They account for around 80% of all species on Earth.