BCD is an acronym for Buoyancy Compensation Device, sometimes referred to as BC or buoyancy compensator.  BCDs are used by divers to adjust their buoyancy, which is done by adding air to inflate the BCD and increase buoyancy or by removing air to deflate the BCD and decrease buoyancy.  Ideally, divers inflate their BCDs underwater to achieve neutral buoyancy.

In warm water, divers generally wear skins or thin 2mm-3mm shortys or wetsuits, which require only a small amount of weight to descend.  BCDs for warm water diving are typically more compact and lower lift capacity.

In colder water, divers generally wear thicker 5mm-7mm wetsuits or drysuits, with hoods and gloves, which can require a substantial amount of weight to descend.  BCDs for colder water diving are typically larger and have greater lift capacity to offset the additional weighting.