Answer:
Mammals are a class of animals belonging to the Phylum Chordata and Subphylum Vertebrata. They are defined by certain characteristics which all mammals share, including: glands that produce milk, hair, three bones in the middle ear, and a neocortex in the brain.
Most mammals are born live, but there are exceptions. Monotremes, including the echidna and the platypus, lay eggs. Most other mammals give birth to live young mature enough to live outside the womb (called placental mammals). Some are born under developed and live for a while in a special pouch until mature enough to enter the world (called marsupials).
There may be some confusion about mammals which do not appear to have hair, such as porpoises. ALL mammals do have hair, at least at some point in their lives. For some it is just a few whiskers or ear hair which may even fall out by the time the animal reaches maturity.