Marsupials belong to the infraclass Metatheria.  The Metatheria, or marsupials are found in Australia and South America, and include kangaroos, koalas, wombats, bandicoots, opossums, phalangers, etc.

Marsupials are generally distinguished by their pouch, although this is not an obvious pouch in all species, the carnivores, for example, have a fold of skin which swells to protect young rather than a fully developed pouch. In many respects, a marsupial is more like the egg laying Monotreme, (i.e. platypus or echidna), than other mammals, except that it’s young are born underdeveloped, rather than out of an egg.  Embryonic marsupials, like the developing embryo in an egg, are connected to a yolk sac, rather than to a placenta as in the more advanced mammals. At birth, they crawl to the pouch and once inside the pouch, attach to a nipple. But as we said before, not only marsupials have a pouch (Echidna also show a marsupium).  

The marsupial brain is relatively small, compared with that of placental mammals. It lacks the connection between the two halves of the brain (hemispheres) that is developed in the Eutheria.  

It is believed that Marsupial ancestors moved from the Americas to Europe in the Northern Hemisphere and to Australia and Tasmania via Antarctica in the Southern Hemisphere, around 65 million years ago, when the three continents were joined.

There are 251 living marsupials organized in 7 orders.  170 of these are indigenous to Australia and nearby islands including New Guinea and Tasmania.  The remaining 51 are indigenous to the Americas.  Most American opossums are arboreal (live in trees), but one (Chironoectes minimus) the “Water Possum” is aquatic, and looks a little like a small otter.

Until recently only one order was recognised by mammalogists, the Order Marsupialia.  Recently though, the order was broken into seven (7) orders, according to dentition and toes skeleton elements.
Marsupials are also grouped non-taxonomically by their distribution in the American continent or Australia.  It is believed that

There are around 250 species of Marsupial in total.  The taxonomic relationships comprise the following groups:

North and South American orders: they are different from Australasian orders because they have paired spermatozoa within the epididymis.

Order Paucituberculata
Family  Caenolestidae includes 3 genera and 5 species of shrew or rat opossums from South America. These animals superficially resemble shrews.   The rat opossums are grey-brown in colour and possess a non-prehensile tail.  Their diet consists mainly of insects.  They differ from most marsupials in that the females do not possess a pouch and males have paired sperm.  Caenolestes spp live in wet high elevation forests of northwestern South America, while the other two species live in the Andes Mountains of Peru (Lestoros inca) and in the Southern islands off the Chilean coast (Rhyncholestes raphanurus). 
Order Didelphimorphia
Family  Didelphoidea encompasses the American opossums, containing over 60 species. These are the marsupials that retain more primitive characteristics compared to the ancestor methaterian. Their habitat ranges from North, to Central and South America, down to Patagonia. Most are opportunistic omnivorous, feeding on a range of foods including insects, fruits, plants and other small animals.  They can range in weight between 10 grams and 2 kilograms and have more teeth than any other land mammal. 

They generally have a pouch, although this may be underdeveloped in some species.  Their tail is prehensile, used for grasping branches when climbing.  Some species show periods of inactivity during the colder months of the year (torpor). This species accumulate fat at the base of their tails to sustain them while inactive. They can be found in a range of different habitats from desserts to tropical forests, being burrowers and semi-arboreal. Male opossums are generally solitary by nature, whilst the females can live in small groups.

The Water Opossum, Chironectes minimus is the only marsupial adapted to water habitats such as freshwater streams and lakes and has an aquatic diet. It has webbed feet, water-repellent fur and both males and females possess a pouch.  The pouch in the females is completely watertight.  Females can dive for longer periods as the young can survive without oxygen for several minutes at a time.  These animals are solitary and hunt at night feeding primarily on fish, crayfish, frogs, shrimp and some aquatic vegetation.

The Virginia Possum, Didelphis virginiana is the largest member of the Order and is found in North America.  It is nocturnal and is well-known for its ability to feign death when faced with an apparent threat.  This is believed to be an involuntary reaction which occurs when the Opossum is faced with an extreme threat. However, opossums can also react quite aggressively when threatened, by screeching and baring their teeth.

Order Microbiotheria
This order contains the family Microbiotheriidae.  There is only one species in this family, the “monito del monte” (mountain little monkey -Dromiciops gliroides) that inhabits dense temperate wet forest of Southern Chile and Argentina. It feeds mainly on insects and small invertebrates on the ground and in trees as well as occasionally consuming fruit.  It is nocturnal and semi-arboreal.  The species is small in size, slightly larger than a mouse, has a well-developed pouch and a moderately prehensile tail which is able to store fat. 
It is believed that this Order is more closely related to the Australian marsupials than the Paucituberculats and Didelphyimerophs. 
Australasian orders

Order Dasyuromorphia
This order comprises the following families:
Family Dasyuridae.  This family includes 61 species, amongst which there are the big Tasmanian devil Sarcophilus harisii, and the small mice-like quolls (Dasyurus spp), dunnarts and antechinuses. Their typical characteristics are 4 pairs pointed upper incisors 3 pairs pointed lower incisors, well developed upper and lower canines, 2-3 pairs upper and lower pre-molars and 4 pairs sharp upper and lower molars. They are therefore well equipped for biting and cutting a carnivorous and insectivorous diet. The marsupium is absent or not well developed. They live throughout Australasia in a range of habitats. 

Family Myrmecobiidae. This family includes only one species, the Numbat (Myrmecobius fasciatus), which lives in arid woodlands in Western Australia. The species is characterised by 50-52 poorly developed teeth which are not used in eating except by the very young. It feeds almost solely on termites and has an extremely long tongue protruding from a pointed snout for this purpose. Numbats don’t have a pouch, their young cling to the mother’s fur while suckling from her teats. As opposed to most other marsupials, Numbats are diurnal, most active in the morning and afternoon, which matches the activity of their prey, the termites.  They usually shelter in logs, burrows or tree hollows.  The individuals are solitary in nature and occupy vast territories, with only male and female territories overlapping.  Females usually give birth to four young which cling to her teats for around the first 6 months of life.

Family Thylacinidae.  This family is comprised of one species, the now extinct Tasmanian Tiger or Thylacine (Thylacynus cynocephalus).  This animal was once distributed across mainland Australia and Papua New Guinea, but was confined to Tasmania thousands of years ago, possibly due to competition with the Australian Dingo.  Thylacines were believed to have become extinct in Tasmania due to hunting, disease, habitat modification and competition with wild dogs.  The last individual died in captivity in Tasmania in 1936.  The Thylacine was the largest of the modern carnvirous marsupials.  It was dog-like in size and appearance, aside from its stiff tail and rear opening pouch.  It was characterised by horizontal stripes running along its back and extremely powerful jaws.  The Thylacine preyed on other marsupials, small rodents and birds.

Order Notoryctemorphia. 
Family Notoryctidae.  Only one genus, Notoryctes is represented in this family and order.  There are two species within this Genus, the Southern Marsupial Mole (Notoryctes typhlops) and the Northern Marsupial Mole (Notoryctes caurinus), which are both found in sandy soils and shrub deserts of Western Australia.  Marsupial moles burrow through the soil in search of larvae and worms. They are blind and lack external ears.  They have a pointed head, strong limbs and large claws to assist with burrowing.  They burrow shallowly through sand and soil in search of food, however it is believed that they dig deeper for sleeping quarters and to house young. Marsupial Moles are solitary animals and can come above ground, unlike other moles.  This usually occurs after rain periods. 

Order Peramelemorphia
Family Peramelidae. This group includes 3 genera and 19 species of bandicoots. Members of this family range in size from the very small Mouse Bandicoot of Indonesia, through to the rabbit-sized Giant Bandicoot which weighs around 4.5kg.  Peramelids are omnivorous and long jawed, eating plant material and small insects and invertebrates. Because of this, their ecological range is wide, occupying a variety of habitats. Their main characteristics are 4-5 pairs of blunt upper incisors; 3 pairs of blunt lower incisors; 1 pair canines; 3 pairs of upper and lower premolars ;4 pairs of upper and lower sharp molars.  The members of this family also have long hind limbs adapted for hopping and running.  The pouch is well developed and opens at the back, an adaptation to their burrowing habit.

Family Thylacomyidae.  This family includes two species of Bilby in the Genus Macrotis.  One of which, the Lesser Bilby (Macrotis leucura) became extinct prior to European settlement.  The surviving Greater Bilby or Rabbit-eared Bandicoot, Macrotis lagotis lives in arid Australia, occupying hummock and tussock grasslands and acacia shrublands.   They are nocturnal, resting in burrows of up to 3 metres in length during the day.  Bilbies are omnivorous, feeding on a range of prey including insects, larvae, spiders, and various vegetative matter.  They are characterised by their very long ears and elongated nose.  They also have strong forelimbs and thick claws for digging. 

Family Peroryctidae.  This family includes 4 genera and 11 species of Rainforest Bandicoots including small mouse type (17 cm) to giant bandicoots from New Guinea which can weigh around 5kg. They are nocturnal, terrestrial and solitary, adapted to life in the wet tropical rainforests.  They feed on insects and plant materials and occupy a wide range of habitats.
Order Diprotodontia.
This is a wide order comprising 11 families and 120 species, amongst which there are the kangaroos, koalas, wombats and other herbivorous marsupials.  They are terrestrial and arboreal, and may feed on plants, nectar or insects. As their name implies, all species are diprotodonts which means that they have a shortened mandible with the first pair of lower incisors enlarged to meet the upper ones. In many arboreal species, two feet toes oppose the other two, making it possible for them to hold to tree branches.