Eutherian mammals include humans and most other mammal species, such as cats, dogs, elephants, dolphins, deer, etc.
Eutherians give birth to a fully developed young.
The fetus completes its development inside the mother's uterus. By contrast, marsupials give birth to a very partially developed embryo, which completes its development outside of the mother's body, but inside her pouch. This tiny, newborn creature must climb from the cloaca up the furry exterior of the mother kangaroo, and into the mother's pouch, where it attaches to a nipple. The joey then leisurely drinks milk for many months, as it physically develops into a recognizable kangaroo. Because of their anatomy with the short forelegs and very long hindlegs, hopping is the most energy efficient mode of transportation for marsupials.