Before buying a SCUBA regulator you should be familiar with the parts of a regulator and their names.
The first stage of the regulator is the part that attaches to the scuba tank.The frist stage takes the high tank pressure air (3000 PSI and above) and drops that pressure to a more manageable pressure called an “Intermediate pressure”. It also - of course, supplies air to the second stage.
The second stage or the part that goes in your mouth takes the air which is now at the intermediate pressure (140 PSI) and drops it down to atmospheric pressure or in other words, it matches the pressure of the water surrounding you so you can inhale.
The Hose connects the first stage to the second stage
Balanced Versus Unbalanced- In simple terms, a balanced regulator breathes pretty much the same at all tank pressures. An unbalanced reg will start to breath more stiffly as the tank pressure drops, especially as the tank pressure nears the intermediate pressure of the first stage. A balanced reg will cost a bit more, but will breathe more evenly over the course of a dive. However, if you’re not watching your gauges, you may get very little inhalation warning that you’re about to suck your tank dry. An unbalanced reg should be a little cheaper, but will start to pull harder towards the end of the tank. Some divers like the fact that the reg is, in effect, telling you you’re running low on air.
Piston vs Diaphragm - This refers to how the first stage (attached to the tank) reduces tank pressure (upwards of 3000psi) to an intermediate pressure (125-150psi), so the second stage (the part that’s in your mouth) can reduce the air to ambient (surrounding) pressure. Both types perform more or less the same and both perfectly great and very reliable for recreational diving. The only difference between the two is-
Piston regulators are basic and simple with less moving parts and have a slight performance edge at depths.
Diaphragm regulators are a little more resistant to cold and water with particles like silt, sand or salt as the internal parts aren’t directly exposed to the water like in the piston type.
The differences are negligible in both types and usually aren’t really a serious consideration when choosing a recreational diving regulator.
Adjustable Second Stages - Some regs come with a knob that can adjustment the inhalation (cracking) pressure, and/or a separate lever to adjust the flow of the air once it’s started. You’ll pay a little more for these features but they’ll allow you to adjust the reg to meet diving conditions, as well as being able to "tighten down" the reg if it starts a slight free-flow.
Ports - Check out the number of high-pressure and low-pressure ports on the first stage and make sure there’s enough for your needs. All regs have at least one high-pressure port (for your pressure gauge) and many have two, allowing you to run an extra pressure gauge or air-integrated computer. Low-pressure ports accommodate regulators, octopuses, BC inflator hoses, drysuit inflator hoses, etc. Be sure there are enough for your particular needs. It’s no sin to have more, but it’s no fun not to have enough.
Warranty and Service - See what the service requirements are. Some manufacturers cover the parts for the first two years. Some have a "limited lifetime warranty" which stays in effect as long as you meet the service intervals. Regardless of which you have, the consumer usually pays for the labor. It’s your responsibility to make sure the reg gets the proper service within the specified time frame.