For one thing, mammals are more often warm-blooded than cold-blooded. Warm--blooded animals are able to regulate their body temperatures, keeping warm from the inside rather than the outside. Female mammals also have mammary glands that are used to produce milk; the mammary glands are present on males, but are not used for much.
Most mammals also give birth to live young - humans, for example, do this. Only monotremes - like platypuses - don’t give birth to live young and lay eggs instead, waiting for the eggs to hatch. Another characteristic of mammals is that they have hair of some kind on some part of their body. They don’t have to be completely covered in hair to be a mammal. Examples of mammals include whales, humans, and bats.
