Cabin v's Cargo hold Pressurisation
Just to expand on the answer from SMOC, Conditioned air enters the cabin/flight deck via the normal airconditioning system. This air flows from the inhabited areas into the cargo compartments via vents to have equal pressure throughout the pressure hull.The pressure is controlled by selecting an altitude at which the aircraft will operate, plus 1000' in most instances to provide a buffer between cruise altitude and selected altitude. The flow to atmosphere is regulated via one or more "outflow" valves, these are then backed up by "safety valves" which will open in the event of the structural pressure limit being exceeded. Before landing, usually before beginning descent, the elevation of the destination airfield is selected and the aircraft will be depressurised gradually on descent to arrive with the cabin pressure at zero differential before any door is opened.