The marsupial's taxonomy is divided into 2 primary groups, namely:

• American Marsupials
• Australian Marsupials

There are on a few American Marsupials, compared to the Australian Marsupials. The order Microbiotheria which belongs in the American Marsupials group, only has 1 specie. This specie is the Monito del Monte (little mountain monkey). It dwells in South America. This specific specie is believed to be closely related to the Australian Marsupials, than any other American Marsupials. 

In both American and Australian Marsupials, there are many small, arboreal species. Arboreal, meaning tree dwelling. Primarily, arboreal species live on trees and are relatively small in size.

For this small arboreal marsupial species, the term Opossums is used for the American Marsupials. While similar small arboreal marsupial species in Australia are referred to as Possums.