How do mammals differ from birds and reptiles?
Short answer: Mammals give birth to live children, while Birds and Reptiles lay eggs.
Birds and reptiles lay eggs which later hatch outside the womb.
Mammals have eggs inside them, but without a shell since it is not necessary. When the egg has developed into a finished creatures, the mammal give birth to a living baby.
Other features that mammals have and the other do not have is fur.
Most mammals do not fly, (with some exceptions, like the bat), and none of them have feathers.
Birds and Reptiles also share dinosaurs as a common ancestor.
All insects (non-mammals) have exoskeletons, meaning that they have soft tissue on the inside and the skeleton where mammals would have skin. Mammals have endoskeletons.