The diving equipment is made of a complex set of different accessories that let the human body feel comfortable, and more importantly survive in the hostile underwater environment.
BCD is the buoyancy control device, also known as BC, the buoyancy compensator. Basically, it is a vest which holds the tank and allows the diver to control the buoyancy. Buoyancy is extremely important in water, as is allows the human body to move naturally without being dragged down, or float. It is pure science: basically it is the force that acts on the human body.
The regulator is mounted on the tank valve. There are two different types: 1st stage regulators and 2nd stage regulators. The first is directly mounted on the tank valve, while the 2nd is a pressure gauge and a low pressure inflator gauge, attached to the BCE inflator. The regulators control the air source. The 2nd stage regulator is the reserve, an alternative source that all divers have as backup for himself or a buddy. A modern regulator will work automatically, giving air when you breathe in. It has a pressure gauge, which informs about the remaining air in the tank.
The suit keeps the body warm and protected from scratches or other injuries. Also, the human body needs protection because the heat is contracted twenty times faster in water that in the air. The wetsuit is usually used in warm water. It is made of neoprene and acts as insulation between the water and the human body. The thickness or the length of the suit generates a higher or lower insulation.
The dry suit is commonly used in cold waters. These are airtight, meaning that they keep the body dry for the entire dire. They keep warmth and dryness, thus acting like incubators.
The mask is a piece that allows better visibility underwater. It comes in various shapes and materials, which seals the face.
The fins allow faster movement underwater, thus travelling for long distances, without wasting energy. They come in open heel and full foot types, and can have various channels and shapes.