Diving procedures can be classified into four main groups according to:

Equipment
Breath-hold (free) diving
This is the earliest form of diving. Free divers from Korea and Japan used this technique to gather mother-of-pearls from the sea floor. This type of diving employs a mask, a snorkel and fins. The duration of each dive depends on the ability of the individual to hold his breath as long as he can.
The first deep diver in the history is said to be Yorgos Haggi Statti. He was  a sponge fisherman who was hired in 1911 by an Italian captain to find the lost anchor of the ship. This small person, suffering from pulmonary emphysema, to everyone’s surprise managed to dive three times to 77 meters deep and complete the task.
Vessel diving
This type of diving is performed with a heavy-walled device such as bathyscaphe, bathysphere and submarine. Another way of vessel diving is with the use of a self-contained armored diving suit. The diver is restricted in his movements and he can only be an observer.
Surface-air diving 
A long umbilical, coming from the surface, delivers air to the diver’s regulator or mouth-piece. The devices used in this type are diving bells, underwater habitats and rigid-diving suits (“heavy-footers”).
Scuba Diving – diving with a gas mixture carried by the diver, wearing an autonomous diving suit. There are two main types of scuba apparatuses – open circuit (the exhaled air goes into the water) and closed-circuit (exhaled air is rebreathed).

Breathing Mixture
Saturation diving
Saturation occurs when the diver’s tissues have absorbed all the nitrogen that they can hold at a specific depth. Saturation diving is essential for the scientist and the working diver who want to spend weeks or months on the bottom in order to complete their tasks. Divers stay in underwater habitats - pressure vessels placed at up to 610 meters (2000 feet) depth.

Level of Proviciency
Professional diving
Professional divers receive payment to dive for military, emergency and scientific purposes.
Recreational diving
Recreational diving is done by amateurs and it does not require specialized training (of course, a basic course in scuba diving is needed). Depths of descending are up to 130 feet with compressed air in order to avoid decompression stops. Recreational diving is always done with a buddy. 

Area
Diving for “on-lookers”
This type is done by amateurs who dive not for profit but because of interest in this activity.
Technical Diving
Technical diving includes procedures such as Ice Diving, Wreck Diving, and Cave Diving.