A diving regulator converts the high pressure gas in your tank to the ambient pressure while diving. 

The terms "regulator" and "demand valve" are often used interchangeably, but a demand valve is a regulator that delivers gas only while the diver is inhaling and reduces the gas pressure to ambient. In single-hose regulators, the demand valve is the second stage, which is either held in the diver's mouth by a mouthpiece or attached to the full-face mask or helmet. 

The performance of a regulator is measured by the cracking pressure and work of breathing, and the capacity to deliver breathing gas at peak inspiration flow rate at depth without excessive pressure drop. For some regulators (when diving in extremely cold water) the capacity to deliver high flow rates at low ambient temperatures without jamming due to freezing is important.