Scuba diving ("SCUBA" originally being an acronym for self contained underwater breathing apparatus, now widely considered a word in its own right) is a form of underwater diving in which a diver uses a scuba set to breathe underwater.
Unlike early diving, which relied 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. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving. Depending on the purpose of the dive, a diver usually moves underwater by swimfins attached to the feet, but external propulsion can come from an underwater vehicle, or a sled pulled from the surface.
Regardless of the dive certification/training organization, all student who wishses to become a scuba diver must complete three compulsory sections of the open water course. These sections is manditory for all student divers to undertake which includes;
1. Theory: The theory portion of the open water course may include book work, DVDs, study questions, online presentation of material, and quizzes. Dive theory covers a range of topics such as pressure-depth relationships, the underwater environment, dive planning, and equipment.
2. Pool Work: During pool (or "confined water") classes, students practice basic diving skills in a shallow, calm dive site such as a pool, bay or lake. Divers practice all the skills that they use on open water dives like ear equalization and mask removal and replacement in this controlled environment before entering the open water.
3. Open Water Dives: Under the watchful eye of an instructor, students apply the knowledge and skills learned in the theory and pool sections of the course on real open water dives.