Buoyancy Control Device (BCD, the vest-like thing) with air tank and regulator + pressure gauge. The BCD is an inflatable vest. Together with the weight belt, the wetsuit and your natural buoyancy, you use it to control your ascending and descending. Mostly, though, you use your lungs as a control device and use the BCD for neutral trim.
The first-stage of the regulator attached to the tank valve. This part takes 200 bar tank air and turns it into 7-10bar medium-pressure air. The medium pressure air is then delivered at ambient pressure through the mouthpiece by the second stage demand valve.