Mammals are different from other classes ( Birds, Reptiles and Amphibians) in the following crucial ways
1. They have mammary glands
2. They have different types of teeth( called heterodonty) to chew their food.
3. They have hair which is quite different from Reptiles.
4. An enlarged brain with a neocotrex.
5. Mammals have better balance due to three additional bones in the middle ear.
Here is a quick quiz to familiarize yourself with mammals.