The wetsuit keeps you warm by letting a small layer of water into it. The water is then warmed by your body temperature.
Scuba tanks contain compressed air that allows you to breathe underwater.
Bouyancy Control Device
The BCD has a harness that holds the tank to your back and controls your buoyancy, which is done via an air bladder. When air is pumped from the tank into the BCD, you become more buoyant; when air is emptied out, you become less buoyant
The regulator takes the compressed air in the tank and adjusts the pressure so that it's breathable. The first stage connects to the tank where it adjusts pressure and routes air to the BCD at an intermediate pressure. The second stage, which is connected to your mouthpiece, reduces it again so that it is at ambient pressure.
Because the human body is naturally buoyant, a diver usually needs weights to descend deeper into the water.
Mask, Snorkel and Fins
The mask creates an air pocket, allowing you to see more clearly underwater. The snorkel is a tube that sticks up out of the water and allows you to breathe natural air when near or on the surface. The fins act as an extension of the foot, increasing the surface area that cuts through the water and propels you forward.