Scuba diving is a form of underwater diving in which a diver uses a self contained underwater breathing apparatus(scuba) to breathe underwater.

Unlike other modes of diving, which rely either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas, (usually compressed air), allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold. Scuba equipment may be open circuit, in which exhaled gas is exhausted to the surroundings, or closed or semi-closed circuit, in which the breathing gas is scrubbed to remove carbon dioxide, and theoxygen used is replenished from a supply of feed gas before being re-breathed.

A scuba diver usually moves around underwater by using finsattached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface.

Source: Wikipedia