For people learning to dive the following is a list of essential subjects to understand:

Basic diving theory:
Diving physics
SCUBA Equipment
Physiology
Diving disorders
Diving signals
Buddy system
Basic water skills:
Finning
Wearing a diving mask
Snorkelling
Shallow free-diving
Basic Aqualung skills:
Preparing the Aqualung
Buddy check
Breathing from an Aqualung
Buoyancy control using the Buoyancy Compensator and the lungs
Ascents and descents
Mask clearing and demand valve clearing
Air sharing
Air sharing ascent
Basic Rebreather skills:
Preparing the Rebreather
Buoyancy control using the Rebreather
Ascents and descents
Mask clearing and mouthpiece draining
Bailing out
Bail out ascent
Diluent flush
Dive planning skills:
Buddy system
Use of decompression tables
Use of Dive computers
Breathing gas requirement calculations
Safe dive site selection
Precautions for night dives and drift dives
Dive leading skills:
Compass navigation
Underwater pilotage
Use of surface marker buoys
Use of decompression buoys
Use of distance lines
Use of diving shots
Decompression stops
Rescue techniques:
Controlled buoyant lift
Towing diver and landing a casualty
In water artificial ventilation
CPR on land
Oxygen first aid on land
Technical diving techniques:
Using Nitrox as a bottom gas
Analysing proportion of oxygen in a breathing gas
Calculating maximum operating depth of a breathing gas
Calculating equivalent air depth of a breathing gas
Using Nitrox as a decompression gas
Planning accelerated decompression
Normoxic Trimix as a bottom gas
Hypoxic Trimix as a bottom gas
Vocational techniques:
Cave diving techniques
Underwater photography
Underwater videography
Underwater archaeology
Marine life identification
Marine biology
Dive group leading skills:
Selecting dive sites using nautical charts
Tides and use of tide tables
Weather influences and prediction
Group rescue management techniques
Dive group safety, prevention and supervision
Underwater search and recovery skills
Underwater survey skills
Logistical skills:
Boat handling and seamanship
Boat navigation and position fixing
Compressor operation
Gas blending
Use of group equipment such as diving shots and decompression trapeezes
Recompression chamber operation
Instructor skills:
Teaching diving theory
Teaching personal diving skills
Teaching group diving, safety and rescue skills
Teaching boat handling, seamanship and navigation skills
Teaching instructing skills

There are four separate types of diving, of which SCUBA (Self Contained Underwater Breathing Apparatus) is the most recently invented.

We will not look at skydiving and aerial diving or other types of diving here, only the types related to swimming under the water.

Diving can involved holding one's breath (free diving, skin diving). This earliest form of diving is still practiced for both sport and for money (e.g., ama divers of Japan and Korea, pearl divers of the Tuamoto Archipelago). The breath-hold diver's compressible air spaces are squeezed by the raised water pressure throughout the dive. Each dive, limited by the individual's tolerance for breath-hold and the risk of drowning from hypoxia, is usually less than 60 seconds.

Diving with compressed air or other gas mixture that is carried by the diver (scuba diving).
There are two principle types of scuba: open and closed circuit. Open circuit vents all expired air into the water, and is the mode used in recreational diving. Closed circuit systems, in which exhaled air is re-breathed after carbon dioxide is absorbed and oxygen added, were widely used before open circuit became available, particularly by military divers who wished to avoid showing any air bubbles.

Diving in a container with strong walls.
Heavy-walled vessels can maintain their inside atmosphere at or near sea level pressure ('one atmosphere' or 'one atm.'), and so prevent the surrounding water pressure from affecting the occupants. Such vessels include: the bathysphere, an unpowered hollow steel ball lowered from the mother ship by steel cable; the bathyscaphe, a bathysphere with buoyancy control so that cable is not needed for descent and ascent; and the submarine, which can travel great distances in any direction under its own power. A modern extension of the one-atmosphere vessel is the self-contained armored diving suit, flexible yet able to withstand pressures at great depth.

Diving using compressed air supplied above the surface.
The diver is separated from the supply of fresh air, which is kept on the surface. Air reaches the diver through a long umbilical, which in its simplest form ends in a regulator and mouthpiece carried by the diver. Devices in this category include caissons (huge spaces supplied with compressed air, employed mainly for bridge and tunnel work), diving bells, and rigid-helmet diving suits. In all these devices the diver breathes air at the same pressure as the surrounding water pressure, and so is at risk for decompression problems (bends, air embolism, etc.) if ascent is too fast.

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