The three types of mammals are the marsupials, the monotremes and the placental mammals. These groups are defined by the way in which their babies develop.
The smallest mammal group is monotremes, which lay eggs. The duck-billed platypus is a monotreme.
Marsupials give birth to babies that are not fully formed, and most marsupial mothers carry their young in pouches until they are more developed. Kangaroos are marsupials.
The largest group of mammals is placental mammals. They give birth to young that are nourished by a placenta before being born. A placenta is an organ that allows a fetus to obtain nutrients from the mother's blood while the fetus develops inside the uterus. Humans are placental mammals.