As large mammals that spend their entire lives in water (only coming to the water surface to breath and occasionally feed), whales, dolphins and porpoises are an important food source for coastal cultures with the specialized equipment and skills required to kill and retrieve large animals in water.
Whales, dolphins, and porpoises are fully aquatic mammals, spending their entire life in the water, and are referred to as cetaceans. They are fully adapted to life underwater and for fast swimming, with an elongated streamlined body, modified hind limbs in a broad horizontal fluke, paddle-like fore limbs, no external ear, nostrils located on the top of the head, and long breath-holding capacities, sometime over 1-hour long. Under their hairless, generally smooth skin, they have a thick layer of fat blubber, which serves as insulation and energy storage. They are large, long-lived, reproduce late in life, and produce few offspring over their lifetime. Most cetaceans use echolocation, emitting and receiving sound to interpret their environment, locate preys, socialize, and navigate.
Reference
Barnes LG: Cetacea, overview. In: Encyclopedia of marine mammals. edn. Edited by Perrin WF, Wursig B, Thewissen JGM. San Diego: Academic Press; 2002: 204-208.