Waterboarding is when a prisoner is interrogated by restraining them on a board, a cloth is placed over their face and then water is poured over the cloth.
The wet cloth clings to the persons face and obstructs the prisoners breathing.
The effect of waterboarding is simulated drowning. However the only difference between waterboarding and actual drowning is that water does not enter the prisoners lungs.
This amateur video on YouTube demonstrates how unpleasant it is to be waterboarded. Their first attempt seems innocuous but when they change the type of cloth and increase the quantity of water the effects on the volunteer are evident.
Memos recently released by Washington detail the use of waterboarding. The New York Times reports that the CIA waterboarded one man 183 time in March 2003.
Although the precise timing of the waterboarding has not been reported it is safe to assume that it would have been systematic. To put that in perspective the man (Khalid Shaikh Mohammed) was probably being waterboarded every 4 hours (six times in 24 hrs) every day for 30 days. The logistics required for this level of interrogation is remarkable and would have required two or even three shifts of CIA operatives and interrogators to maintain the torture schedule.
US Army Field Manual (PDF)
The charges against the accused (US Military tribunals)