Basically, a pressure bladder is any bag-like object that can stand up to a pressure differential between its outside and its inside. Some things that qualify as pressure bladders include balloons, whoopie cushions, inner tubes, urinary bladders, volleyballs, sausages, and so on.
In space suit design and similar pursuits, a pressure bladder must be employed as one of the innermost layers of the suit. It is usually constructed of some thin, strong, non-porous fabric, such as coated nylon. The pressure bladder is the part of the suit that is actually inflated, and it is required to stay inflated during use at a certain desired pressure (sometimes against vacuum) without bursting.
The pressure bladder is made just slightly bigger than the next layer of the suit, the restraint layer. This allows the pressure bladder to expand only to the size of the restraint layer, preventing it from bursting or unduly stressing its seams. And of course, by stopping the expansion of the pressure bladder, the restraint layer can shape, mold, and limit the pressurized volume available within the suit; for example, if joints are built into the restraint layer, the pressure bladder can expand to fill those joints, providing a range of motion that would not be available were the pressure bladder inflated to its full extent.
This discussion is based on experience with designing a space suit analogue meant for underwater testing; the basic design of a pressure bladder or pressure bladder/restraint layer pair can be expanded for any number of uses, I'm sure.
Can I make this node clearer? /msg me!