Monotremes, marsupials and placental mammals differ in superficial details relating to teeth and bone structure, but the main difference is in modes of reproduction. Placentals, or "Eutheria," gestate their young internally; marsupial, "Metatheria," offspring mostly develop outside the mother's body and monotremes, "Prototheria," lay eggs.
Monotremes diverged from all other mammals around 190 million years ago and have developed in reproductive isolation since the early Jurassic Period. They do not have teeth, but chew their food with either a hardened bony palate or toughened gums. They have bony processes from the pelvis that run along their ventral surfaces, and their sexual and excretory organs both open to a single aperture called a cloaca.
Marsupials diverged from placental mammals approximately 100 million years ago during the Cretaceous Period. They have specialized teeth, though with a variable number of incisors, and most possess the bony process of monotremes. About half of marsupials have abdominal pouches, which are used for incubating their profoundly underdeveloped offspring. Marsupial wombs and birth canals are biphid, and males' penises are likewise split in two.
Placentals are the largest group of living mammals. Their teeth are diversified, but follow a regular pattern of three incisors, one set of canines and molars at the back of the jaw. Eutherians have no ventral bone process and incubate their young internally. There is some overlap of urinary and genital tracts among placentals, but the rectum is distinct from the urogenital tract.