Odontoceti, AKA the toothed whales, are the group of whales that have teeth -- as opposed to baleen, as found in the Mysticeti. The Odontoceti includes dolphins, porpoises, belugas, narwhals, beaked whales, and sperm whales. As evidenced by the narwhal and the beaked whales, the toothed whales sometimes have some impressive tusks, but in most cases the teeth are more traditional.
In general, the Odontocetes are smaller and faster than the baleen whales; obviously, one of the major benefits of having teeth is to eat things larger and crunchier than krill and plankton, and the Odontocetes include a number of fearsome hunters (the sperm whale and the orca, for example). Their diet tends to include fish, squid, crustaceans, and pinnipeds.
Another adaptation to aid them in hunting is echolocation; the baleen whales do not echolocate, but it is believed that all toothed whales do. In most cases, members of the Odontoceti emit a highly-focused beam of high-frequency clicks; these sounds are reflected forward by the dense, concave bone of the skull, and in many species the sound is modulated by a large fatty organ known as a "melon", found on the forehead. The melon consists of lipids of varying densities, and acts like an acoustic lens to focus the sound. The clicks are produced through the blowhole and a system of nasal sacs; Odontoceti do not have vocal cords. In some species (such as the dolphins), the returning echos are received through the jaw, where the jaw bones and teeth act as a amplifying antenna.
Due to evolutionary fine-tuning of the nasal sinuses and jaw structure to maximize echolocation, Odontocetes have slightly asymmetric skulls; the right side of the skull is slightly larger, and in some species the right jaw may be smaller than the left side. This is considered to be a defining feature of the Odontocetes, and asymmetry in fossil skulls is considered a reliable sign that a species had developed echolocation.
Other characteristics of the Odontoceti that distinguish them from the Mysticeti include: a single blowhole, where the baleen whales have two (the second blowhole has sealed up, although the airway beneath remains patent); fused lower jaws, providing more support for biting; comparatively large brains, which evolved in conjunction with echolocation; and the absence of any olfactory sense. Odontoceti also tend to have better vision than baleen whales, although sperm whales have poor eyesight, and the Ganges river dolphins are completely blind.