Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and porpoises. Dolphins and porpoises may be considered whales from a formal, cladistic perspective. Whales, dolphins and porpoises belong to the order Cetartiodactyla, which consists of even-toed ungulates. Their closest non-cetacean living relatives are the hippopotamuses, from which they and other cetaceans diverged about 54 million years ago. The two parvorders of whales, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have had their last common ancestor around 34 million years ago. Mysticetes include four extant (living) families: Balaenopteridae, Balaenidae, Cetotheriidae, and Eschrichtiidae. Odontocetes include the Monodontidae, Physeteridae, Kogiidae, and Ziphiidae, as well as the six families of dolphins and porpoises which are not considered whales in the informal sense.

सीटॅसिया या वर्गातील सर्व जलचर सस्तन प्राण्यांना देवमासा म्हणतात. सीटॅसिया वर्गातील सर्वांना जरी देवमासा म्हणतात, तरी बऱ्याचदा Odontoceti या उपवर्गातील डॉल्फिन व गाधामासा(डॉल्फिन सारखाच, ज्याला इंग्रजीत पॉर्पॉइज म्हणतात) यांना त्यातून वगळण्यात येते. या उपवर्गामधे स्पर्मव्हेल, किलरव्हेल, व बेलुगा व्हेल हे देखील आहेत.