There appear to be two different varieties of torture referred to as waterboarding. The first of these also goes by the name of the water cure.
In the second form of waterboarding a victim is strapped to a board and lowered into a vat of water until he or she believes that drowning is imminent. The subject is then removed from the water and revived. If necessary the process is repeated. The torture is designed to be psychological more than physical, as the victim is led to believe that he or she is being executed. This reinforces the torturer's control and makes the victim experience mortal fear.
A similar torture was applied to supposed witches. In a trial by ordeal called "dunking," they were immersed into a vat of water or pond, and taken out after some time, when the victim had the ability to confess. If she confessed, she was killed. If she did not confess, she was submerged again. This process usually was repeated until the victim drowned or gave up and let herself be executed in another way (hanging or, rarely, burning).
In June 2004, it was revealed that the United States Department of Justice had written memos stipulating that torture was not illegal unless it was intended to cause extreme physical pain, and it was further revealed that "waterboarding" was being used on prisoners at Camp X-ray in Guantanamo Bay. There is evidence that water boarding was recently used in Abu Ghraib and was one of the forms of torture used on those detained there.
Waterboarding is also used by the US military on US troops that have a high probability of capture. This training exercise is to help familiarize the soldiers to the sensation so that they are not unprepared if captured. Jet fighter pilots are also subjected to similar experiences where they are dumped with great force into deep pools of water and left to unbuckle their harness and find their way to the surface while upside down and wearing full flight gear. The fear induced in pilots during this training phase was well captured in the movie An Officer and a Gentlemen.