A monotreme is a mammal that lays eggs. This means that they do not give birth to live young. They also keep the eggs inside for a longer period of time to provide nutrients to the egg. An example of a monotreme is a platypus or echidna.
A marsupial is a mammal that has a pouch in which to carry their young. Marsupials generally give birth earlier that placental mammals. They have very strong limbs because they have to climb to the mothers nipples to feed. An example of a marsupial is the koala and kangaroo.
A placental mammal is a mammal that is nourished in the mother's uterus and born developed. This is a very diverse group and consists of whales, cats, dogs, and humans, just to name a few.
Monotremes are mammals that lay eggs. Monotremes are warm blooded with a fast memetabolism. They have hairy bodies and produce milk in their mammary glands.
Marsupials are mammals that give birth to live young. These mammals have a pouch where their young grow and develop. Marsupials are hairy, warm blooded, and produce milk. One of the biggest differences between marsupials and placental mammals is that marsupials give birth quite early and rely less on the nourishment of the placenta. Some examples of marsupials are kangaroo and opposums.
Placental mammals are mammals that give birth to fully developed live young. They differ from marsupials in that the baby spend more time being nourished in-utero by the placenta. These mammals are hairy and warm blooded as well. Some examples are mice, rats, and bats
Monotremes are animals that lay eggs, such as a platypus. While they do produce milk for their young, like all mammals do, the milk is secreted out of the skin on the mother's chest (they do not have nipples). Platypus are also unique mammals in that the males have venemous talons on their rear legs.
Marsupials are animals like kangaroos and wallabies, that do have live young but a very short gestation period. The young are very small when they are born, and must crawl to the mother's pouch on their own right after being born (the only aid the mother gives is that she licks a trail of saliva for the young to follow). The young then attach to a nipple in the mother's pouch, where they continue to grow. Marsupials have a bicornate uterus - it basically has two sides. This allows them to mate and fertilize two eggs - one on each side. Only one zygot will develop and crawl to the pouch. If that offspring does not make the journey (which happens fairly often), the other fertilized egg will develop.
Placental mammals are all the other mammals - they have live young (do not lay eggs) and do not have a pouch. This includes humans, cats, horses, elephants, dolphins, etc.