Dolphins are mammals, not fish. They, along with porpoises and whales, are cetaceans — marine mammals belonging to the order Cetacea. Unlike fish, dolphins are warm-blooded, not cold-blooded, and breathe using lungs instead of gills.
Dolphins and other cetaceans use a blowhole at the top of their heads to inhale air. This requires dolphins to surface every time they need to inhale. Like other mammals, dolphins give birth to live young, whom the mothers feed with milk. Fish, on the other hand, reproduce by laying eggs. As of 2014, studies indicate that cetaceans evolved from land-dwelling mammals that returned to the ocean.