Traditional Personal Flotation Devices use inherently buoyant materials, such as foam, to stay afloat. Inflatable PFD's, as their name indicates, rely on inflatable chambers that provide buoyancy when inflated.
Un-inflated, these PFD's are less bulky than inherently buoyant PFD's. Inflatable PFD's come in a variety of USCG defined Performance Types. The specific Type of PFD is determined by characteristics such as its amount of buoyancy, its in-water performance and its type of inflation mechanism. To understand the details of your PFD, read the PFD label and owner's manual.