Marsupials are non-placental mammals in the order Marsupialia. Because marsupials do not develop placentas like other animals, their young are born prematurely, at which point they are incubated in a pouch known as the marsupium. Many people are familiar with Australian marsupials like koalas, kangaroos, and wombats, and marsupials can also be found in the Americas, primarily in the form of opossums. All told, there are around 260 individual marsupial species.
According to the fossil record, marsupials evolved at around the same time that placental mammals did, and marsupials were once widespread in many parts of the world. Over time, marsupials were pushed out by placental mammals, who presumably developed traits which allowed them to survive more successfully. It is theorized that marsupials did especially well in Australia and the surrounding areas because of the relative isolation of the Australian continent, which allowed marsupials to thrive.
When placental mammals breed and gestate young, they create what is known as a placenta, an organ which supplies the developing embryo with nutrients from the mother's blood and filters toxins. Marsupials instead develop a yolk-like sac which nourishes an embryo for a brief period of time after fertilization. When the embryo runs out of nutrients, it is born in an extremely premature and very underdeveloped state, looking like little more than a shrimp.
The embryo of a marsupial is too delicate to face the outside world, so instead it heads for the marsupium, where it latches on tightly, receiving nutrients in the form of milk and growing into a full term baby. Many young marsupials continue to live in the pouches of their mothers for some time after they are able to face the world, spending more and more time out of the pouch until eventually they branch out on their own.
The process of reaching the marsupium is quite difficult, as the embryo must climb up its mother to reach the pouch. As a result, the embryos have extremely well developed forearms which they use to hoist themselves up. Once they latch onto the pouch, they are often so tightly attached that they cannot be removed without injury; people initially believed that marsupials were gestated entirely in the pouch as a result, arguing that the embryo was obviously firmly attached to the wall of the pouch.