Most animals that live on land lay eggs that are protected by hard shells. They fertilize their eggs internally. Water-dwelling animals like fish and amphibians lay eggs that have not been fertilized yet. Then the males lay sperm on top of the eggs, which is called external fertilization.
No mammals except for monotremes reproduce like this. Instead, they are viviparous. Fish, amphibians and reptiles that are not oviparous are oviviparous. This means that the eggs hatch while still inside the mother's body.