There are many different types of valves and connections that you can come across when scuba diving.
A-Clamp or DIN
Most recreational divers will come across two types of connections that allow them to attach a first-stage to the cylinder.
The first type is A-clamp (yoke) and this is the most common type of the connection. This surrounds the the post of the valve and traps the o-ring of the valve against the first-stage of the regulator. It has a maximum rating of 200-240 bar. The o-ring can be easily damaged by over pressurisation. It is possible to use an A-clamp on a DIN tank if you use an insert on the tank.
The second and less common type of connection is the DIN. This connection screws into the post of the valve, securing the o-ring inside the post. This makes the o-ring better protected than the A-clamp, it is also less likely to have a problem if it is knocked. The DIN valve comes in two different pressure ratings 200 and 300 bar. It is possible to get an adaptor to use a DIN regulator on a tank that is set up for A-clamp.
The most common valve that recreational scuba divers see is the K-valve. This allows one regulator to be attached to the tank either by an A-Clamp or DIN connection.
Prior to the K-valve was the J-valve. The J-valve would restrict the air flow when the tank was low, the diver would then pull a lever and this would release a reserve of air. These valves were used when submersible pressure gauges were unavailable/uncommon, however once SPGs become available they fell our of favour.
With more advanced diving there are three different valves that can be also used. There is the H-valve, the Y-valve and the manifold (with or without an isolator). The H and Y-valves are similar. They have two outlets and allow two first-stages to be attached to a single tank. The Y-valve has two dip tubes which means that it is possible to shut down one of the attached regulators, while keeping the other open to breathe from. Why would you want two first-stages? When diving in cold conditions regulators can sometimes freeflow so having a secondary regulator would give you a back up, also in an out of air situation at depth having a secondary regulator will mean that each diver won’t be fighting for air.
A manifold is used on a twinset. It connects two tanks (before the regulators) and makes them into one tank, two regulators are usually attached with one second-stage on each first-stage. Often technical divers will use this set. The addition of an isolator allows the tanks to be made independent.
I currently use Dive Rite left and right handled valved on my tanks. They are DIN valves that when combined with an insert can be used with an A-clamp. My regs are all DIN as that is, in my opinion, a safer setup as the o-ring is trapped inside the post of the valve. Also it makes the first-stage smaller meaning that it is less likely to be knocked when I am in a confined area (such as a cave) and if it is knocked it is less likely to be damaged.