Theria is a subclass of mammals that includes all living mammals except for monotremes, which are egg-laying mammals like the platypus and echidna. Theria is divided into two main groups: Eutheria, or placental mammals, and Metatheria, which includes marsupials like kangaroos and koalas. Therians give birth to live young, which typically develop inside the mother's womb for a period of time. This reproductive strategy allows for more complex development before birth, contributing to the diverse range of species found within this subclass, including humans, whales, and elephants.