Virtually all scuba cylinder valves are made from chrome-plated brass. Historically, divers identifies cylinder valves as two basic types: the K-valve, which is a simple on/off valve, and the J-valve, which has a built in mechanism that signals when you run low on air.
The J-valve contains a spring-operated shutoff valve that is held open by cylinder until the pressure drops to approximately 20-40 bar/300-500psi. When the cylinder pressure drops below that point, the pressure no longer holds the shutoff open, causing breathing resistance to increase and warning that air is low. Pulling down the reserve level releases the remaining "reserve" air.
Although J-valves wee almost standard equipment in the 1960s before common use of submersible pressure gauges, today you see them mush less often and usually left in the non reserve position. An exception is in areas where regulations require them. They're prone to accidental tripping (so they don't warn you), and they increase the cost and service requirements of the valve. The only reliable way to monitor your cylinder pressure is to use a submersible pressure gauge, (SPG).
Today, you can identify cylinder valves as yoke valves in DIN (Deutschees Institut fuer Normung) valves. By faer the most common are yoke valves; as the name implies, you attach the regulator via a yoke assembly.
With the DIN valve system, you screw the regulator into the valve. Although less common worldwide, the DIN valve system has the advantage of being rated to higher working pressures. The DIN system is very common in central Europe.
One thing to notice is that all cylinder valve connections with the regulator require an O-ring, which makes an air tight seal. You find the O-ring mounted in the air valve with the yoke system, and mounted in the regulator with the DIN valve system. Either way, you can't dive without this O-ring – the regulator would not seal so learn to check for it when setting up you dive equipment.
Another feature you find in the valve is the burst disk. Burst disks relieve cylinder over-pressurisation which can happen by accidentally over filling the cylinder or by exposing it to excess heat. If the pressure gets too high, the burst disk ruptures, releasing the air well before the cylinder would explode. In some countries, cylinder valves do not have burst disks.