Scuba diving regulators (regs) are the most important part of your kit - without a regulator Scuba wouldn't be possible! Your regulator is your life source so you want to make sure you choose a great quality reg from a reputable brand. Purchasing a brand new reg is a better option than going for a second hand set. When purchasing second hand, ensure the kit has a full service history.
The scuba regulator receives high pressure air from the tank through a yoke or a DIN valve and adjusts it to the pressure required for breathing. It supplies oxygen only when you inhale (on demand) and directs your exhalent Carbon Dioxide into the water. It also routes air to your BCD for buoyancy and instruments including a pressure gauge to see how much air you have left.
The safety Octopus is a back-up regulator, to be used in emergencies only. The hose is coloured bright yellow to enable your buddy to locate it easily.