What are the three subgroups of mammals and their features?

Mammals are classified into three subgroups: monotremes, marsupials, and eutherians (placental mammals). Monotremes, like the platypus, lay eggs and produce milk without nipples. Marsupials, such as kangaroos, give birth to underdeveloped young that continue to develop in a pouch. Eutherians have a complex placenta that nourishes the young during gestation, allowing for longer developmental periods before birth.