|Possums, opossums, or both?|
|Common brushtail possum (Trichosurus vulpecula), native to Australia|
Suborder Phalageriformes is included in the Order Diprotodontia, one of the many Orders to be found in Class Mammalia, and one of seven orders of marsupials. Order Diprotodontia includes the largest number of marsupial species, including wombats, bettongs, and the ever-charismatic kangaroos and koalas. Often, a diprotodont will be the first thing someone thinks of when they hear the word "marsupial."
|Elegant fat-tailed mouse opossum (Thylamys elegans), native to Chile|
What are some of the differences between possums and opossums? Beyond a number of morphological differences, possums are mainly herbivorous, nectivorous, or insectivorous while opossums are mostly omnivorous or carnivorous. This ties into the morphological differences, as the teeth of the two groups are quite different.
It remains true, however, that opossums are colloquially referred to as possums by many people. Some consider all of these species to be possums, with just the American species being opossums. However, since there are significant differences between the two groups I don't think this is appropriate.
Sources are Animal Diversity Web and Merriam-Webster. Images are from Wikimedia Commons under Creative Commons licenses: one, two, three.