Dolphins are cetaceans which are a group of mammals fully adapted to live in the water. While most species live in the oceans and seas, few species dwell in rivers.
However, dolphins are an informal group which is not self-contained taxonomically. They belong to the family Delphinidae which includes the oceanic dolphins, but this group also includes the river dolphins.
Like all mammals, they have warm blood and breathe through lungs. Dolphins are one of the most intelligent animals in the world that have excellent communication capabilities.