What is a Mammal?

Mammals, despite there only being about 4,000 known species, are a complicated group of animals that can actually be subdivided in 3 sub-classes; Monotremes, Marsupials and Placental Mammals.
Monotremes are extremely rare-there are only 2 species left that represent this group: Duckbilled Platypus and Echidnas. Unlike other mammals which give birth to live young, montremes actually lay eggs.
Marsupials are mammals whose young are born extremely under-developed and complete their growth within their mother’s pouch. Animals like Kangaroos and Koalas are marsupials.
Placental mammals are the most common group of mammals. The young are sustained within their mother’s womb by an organ know as the placenta. Babies are born fully formed, but all require continued parental care. Examples of placental mammals are lions, monkeys and dolphins.
Regardless of which sub-class of mammal it falls into, ALL mammals receive milk from their mothers as infants, are warm-blooded and have hair/fur at some point in their lives. 

Marsupials:

Placental Mammals: