BCD's: Buoyancy Control Devices (aka Buoyancy Compensators)
Factors to consider when buying,or even renting a BCD include fit; style of the device (jacket or back-inflate); travel weight; lift; weight pocket design; tank plate and strap design; BCD pockets; number of D-rings; dump valves; alternate air source; and durability.
"Fit" first means that the BCD should be long enough to allow the weight to sit on your hips and to make sure the chest strap doesn't choke you! How it fits around you can be adjusted.
"Style" is about how the BCD positions a diver with some to a lot of air in the bladder. A back-inflate BC will push the diver forward into a comfortable diving position, streamlined horizontally. A jacket-style BC supports you better upright on the surface. Dive the BC before you buy to discover which you prefer.
"Travel Weight" ranges between four to eight pounds. To reduce the total weight of the BC, manufacturers remove the heavier parts of a full BC and replace the parts with plastic or not all all. Common adjustments include a reduced tank plate, velcro straps instead of metal buckles; a vastly reduced or eliminated cumberbund; and smaller to no pockets.
"Lift" refers to the capacity of the BC to create positive buoyancy when the diver is wearing additional weight. If you are diving in cold water, you will wear additional layers of neoprene. That, in turn,requires additional weight. The BC must be able to compensate for the loss of that buoyancy at depth.
"Alternate Air Source" means you can carry your safe second or octopus on the inflator hose of your BC. The addition to the BC raises cost but eliminates the need for a secondary second stage - fewer hoses to track!
As is true of most scuba purchases, try your BC of choice in the water before you spend the money.