They are cetaceans, members of the whale family. They breath through a blowhole on top of their head, spouting water from it, like any other whale. Dolphins are air-breathing mammals, despite their shark-like fin, whereas sharks are fish and breathe in water. Dolphins give birth to live young (calves), raising them on milk. Sharks lay eggs. Cetaceans' fish-like appearance is a case of convergent evolution, they have developed the shape to suit a life spent entirely in water. Around 100M years ago there was a creature called the ichthyosaur. It too resembled a fish or shark, but it was actually a reptile.