Technically, only mammals give birth to live young. This means they develop inside the mother's body and are born as relatively complete individuals.
However, some animals, like certain reptiles and fish, are ovoviviparous. This means their eggs hatch inside the mother's body, and the offspring are born live.
So, while mammals are the only animals that truly give birth to live young in the traditional sense, other animals may appear to be giving birth live even though their offspring actually hatched from eggs inside the mother.