These ships have been developed from the older and perhaps more famous passenger liners that once offered scheduled services for passengers across the oceans of the world before the advent of air travel.
The main differences between passenger liners and cruise liners are:
Of course, many ships that were originally built as passenger liners are now in operation as criuse liners, such as Cunard's QE2, the Norway (formally France), Switzerland for example.
One of the Costa Line cruise ships taking on new passengers at Genoa. Note the "boxy" nature of the design. This is to maximise the accomodation for the size of hull.