Mammals - Monotromes Marsupials Placental Mammals

Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales)~`
Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crowngroup Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.
Also all mammals, other than the monotremes, give birth to live young instead of laying eggs. Most mammals also possess specialized teeth and use a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart

Mammals with pouches are called marsupials.Like other mammals Marsupials give birth to life young,But they give birth to young that are still very tiny and unformed.
Mammals which lay eggs are called monotremes. Platypuses and echidnas are the only monotremes.
Monotremes are mammals that lay eggs.They are only two monotremes: platypus and echidna.
Placental mammals are a rather diverse group, with nearly 4000 described species, mostly rodents and bats.