In sexual reproduction, a sperm unites with an egg in a process called fertilization. The fertilized egg develops into a new individual. In all species of mammals, the eggs are fertilized inside the female's body. Mammals can be divided into three groups according to the way in which the fertilized egg develops into a new individual. These groups are placentals who give birth to fairly well- developed offspring.
Marsupials give birth to poorly developed young.
Monotremes lay eggs rather than bear their young alive. Here some examples of different kinds of mammals. The Sea World Book of Whales. Sea World Press, 1980. Kangaroos and other Marsupials.