Regulators or regulator systems are really composed of many parts. The stage that attaches to the tank is known as the first stage, then comes the long air hose. This in turn is followed by the 2Nd stage, the part that actually goes into your mouth so you can breath the air.
First stage role is to accept the 3000 psi cylinder pressure coming from the tank and reduces it to an ambient pressure plus a preset intermediate pressure. It is here at second stage that its role is to reduce this ambient-plus-intermediate pressure to simply an ambient pressure so you can breathe off it. At this point we will not go into great detail on the exact mechanisms involve with this reduction.
Some first stages offer environmental sealing. This is where the first stage is completely sealed off from the water and air. Where as non-environmentally sealed regulators are open to the water temps and debris.
There are many ways to group regulators: piston vs diaphragm first stages, balanced vs unbalanced, downstream vs pivot valves, open vs closed. Even on these listed there are many combinations. Just understand there are many variations.
Still another way is what I will call non-sealed and sealed regulators. You can see on the non-sealed there are small holes on the sides, where as on the sealed the holes are missing. This is the easliest way to tell the difference.
In the sealed regulators, the water pressure doesn't act directly on the piston or diaphragm. Instead it acts on the silicone or alcohol based fluid inside a watertight, yet pressure-sensitive, barrier. This fluid then transmits to the piston or diaphragm.
Why is this important in diving ? First, it prevents salt, sediments and other contaminates from entering the first stage. This reduces internal corrosion and contaminant buildup. Second, it helps isolate the valve mechanism from freezing. This is important if you dive in extreme cold water, like in ice diving. If the first stage does freeze while diving it will seize to work.