Marsupials reproduce and give birth to a partially developed offspring, whereas mammals give birth to a fully developed offspring.
Mammals and marsupials are both warm-blooded and feed their babies with milk. However, mammals give birth to a fully developed baby while marsupials give birth to a partly undeveloped offspring that must be put in a pouch for full development. Also, mammals have only one sex organ while marsupials have two (male and female).
Example of Marsupial: Kangaroos put babies in their pouches for full development.
Example of Mammal: Whales give birth to fully developed babies.