The practice grew gradually out of the transatlantic crossing tradition, which despite the best efforts of engineers and sailors into the mid-20th century, rarely took less than about four days. In the competition for passengers, ocean liners added many luxuries - most famously seen in the Titanic, but also available in other ships - fine dining, well-appointed staterooms, and so forth. With the advent of air travel, nearly all travellers switched, but there were some who actually enjoyed the few days of enforced idleness, so while the ocean liner business crashed, the voyages never stopped altogether.
As of 2004, several hundred cruise ships, some carrying over 3,000 passengers, and displacing over 100,000 tons - placing them among the largest ships ever built - ply routes all over the world. For certain destinations, such as Antarctica, cruise ships are very nearly the only way for tourists to visit.
Present-day cruise ships are organized much like floating hotels, with a complete "hospitality staff" in addition to the usual ship's crew. It is not uncommon for the most luxurious ships to have more crew/staff than passengers.
Many older cruise ships have had multiple owners over their lifetimes. Since each line has its own livery and often a naming theme (for instance, Holland America ships have names ending in "dam"), it is usual for the transfer of ownership to entail a refitting and name change. Some ships have had a dozen or more identities.
Cruise lines (note that many of these are now subsidiaries of Carnival Corporation):