Please distinguish between monotremes, marsupials, and placental mammals.

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.