Here's a breakdown of the three categories:
* Placental mammals: These are the most common type of mammal. They give birth to live young that have developed inside the mother's body, connected to a placenta that provides nourishment and oxygen. Dogs, cats, humans, and most other familiar mammals belong to this group.
* Marsupials: These mammals give birth to relatively undeveloped young that continue their development in a pouch on their mother's body. Examples include kangaroos, koalas, and opossums.
* Monotremes: These are the most primitive mammals. They lay eggs instead of giving birth to live young. The only living monotremes are the echidna and platypus.