Regulators are the life support system for Scuba Divers. The first stage of the regulator connects directly to the tank via a yolk or a DIN valve. The first stage takes the high pressure air from the tank and sends it to the high pressure hose of the pressure gauge or transmitter, as well as converts the high pressure air into intermediate pressure which is sent through the hoses to the low pressure inflator, primary second stage, and alternate air-source. The second stage and the alternate air-source are the parts you put in your mouth, which converts the intermediate pressure to ambient pressure suitable for breathing.
First and Second stage:
First stage regulators are offered in a wide range of designs, types, and offer many different features. First stages are available in yolk and DIN configurations offering features such as environmental sealing to keep the regulator from freezing in cold water. Different first stages also offer different numbers of ports available for connecting hoses and transmitters. Small and travel regulators offer only one high pressure port meaning that the diver must choose between a sole transmitter or a pressure gauge. Many higher end first stages offer multiple high pressure ports for both pressure gauge and transmitter use.
Second stages offer the air directly to the diver and are kept in the mouth during the dive. They are offered with many different features such a surface and dive modes, to make the work of breathing easier as you descend. They are also made in different sizes and of different materials so they will feel different when sitting in your mouth.