Now the actual tonnages, as a measurement of weight:
Displacement is, as the name implies, the weight of the water that the ship displaces at various times, which equals the weight of the ship, incl. cargo etc.
Loaded displacement is the weight of the ship when loaded to her max. allowed Load Line, while Light Ship displacement is the weight of the hull and machinery only.
The difference between them is the Deadweight Tonnage, or the max. weight the ship can carry in the form of Cargo, Bunker Fuel, Water, Stores and the weight of Crew and Pax and their belongings, especially if a Cruise ship.
For obvious reasons the Deadweight of a Cruise Ship is not a meaningful measure of her capacity. Neither is Gross/Net Tonnage, or even Displacement in most cases, but it is the best we got, aside from max. number of Pax and her length over all.
Hope this helps, but it probably should have been under a different thread, as it doesn't do much for the QE 2.