The main difference is that mammals give birth to their babies and after that they feed them with milk. This means that mammals have mammary glands, where the milk appears when the baby(es) born. The reptiles instead lay eggs, and when the eggs open, the little ones are ready for the life, they don't need any milk and can already eat the things that adults eat.
Another difference is the body heat: mammals can produce body heat, while reptiles need a source, such as the Sun (this is why we often see lizards or other reptiles chill on a rock in a hot summer day).
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