Here's why:
* Mammal: Possums belong to the class Mammalia, which means they are warm-blooded, have fur or hair, and nurse their young with mammary glands.
* Marsupial: Possums are specifically members of the order Diprotodontia, which are marsupials. This means they give birth to relatively undeveloped young that continue to develop in a pouch on the mother's abdomen.
So, while all marsupials are mammals, not all mammals are marsupials. Possums fit into both categories.