Regulators
 
Regulators enable you to use the air inside your scuba tank. It reduces the scuba tank's high pressure air to match the surrounding water pressure, and it delivers air only on demand, when you inhale. It regulates the air flow, hence the name "regulator". Sometimes it is also known as a "demand valve".
The piece of equipment, referred to the "first stage", called a piston is responsible for reducing the high tank pressure to an intermediate pressure of 7-10 bar/100 to 150 psi above the surrounding water pressure.
Here are a few examples of regulators:
 
Delta II Sub Zero by Oceanic
Delta II Sub Zero by Oceanic
Gamma Nitrox by Oceanic
Gamma Nitrox by Oceanic
 
Here are a few examples of pistons :
DX4 Diaphragm by Oceanic
DX4 Diaphragm by Oceanic
 
SP4 Piston by Oceanic
SP4 Piston by Oceanic
 
for more information on regulators and pistons, please visit these sites :
Oceanic
Mares