BCD (Buoyancy Control Device)
The BCD is considered standard mandatory scuba equipment; you cannot go diving without one. BCD stands for buoyancy control device which is, as you can imply from the name, a device which allows you to control your buoyancy. In the simplest terms, it is an expandable bladder that you inflate or deflate to regulate how well you float, in this sense it is very similar to a fish's swim bladder. You can do this orally using air from your lungs, though most of the time you'll use a low pressure inflator, which inflates the BCD with air directly from your cylinder. To deflate the BCD, in order to reduce buoyancy and help you sink, you use a hose or valve connected to it.
You might wonder why it is important to be able to control your buoyancy. Besides allowing you to regulate your buoyancy underwater, which is useful for ascending or descending to different depths, it can provide positive buoyancy at the surface. This is vital for resting, swimming or lending assistance to others. If you had to do those things whilst carrying heavy equipment such as a cylinder and kicking or treading water the entire time, you would be too exhausted to dive.
There are three basic BCD styles; front-mounted, back-mounted and jacket-style. Of these, the far most common used by both professional and recreational divers is the jacket-style. The reason the other styles have fallen out of popularity is that they're more difficult getting in or out of and some also require you to have a separate backpack for the cylinder, where the jacket-style has this already integrated.
You still see some back-mounted BCDs (see picture), also referred to as 'wing style' used for double cylinder diving, but again these are relatively uncommon these days and tend to only be used by technical divers who really know what they are doing. Plus points of this particular style are that it keeps your front area free from clutter and packs down to a more manageable size.
The jacket-style, much more popular and easier for recreational diving, wears like a sleeveless coat holding your cylinder in place as well as keeping you afloat. It inflates both in the back panel and in the side panels which wrap around your torso making is favourable to recreational divers as their buoyancy is even. The back-mounted style only inflates in the back panel meaning you tend to be thrusted face first into the water.