A passenger ship is a ship whose primary function is to carry passengers.
An ocean liner is the traditional form of passenger ship, apart from smaller craft used for coastal voyages and as ferries. In the latter part of the 20th Century ocean liners gave way to cruise ships as the predominant form of passenger ship.
Although some ships have characteristics of both types, the design priorities of the two forms are different: ocean liners value speed and traditional luxury while cruise ships value amenities (swimming pools, theaters, ball rooms, casinos, sports courts, etc.) at the expense of speed. These priorities produce different designs. In addition, ocean liners typically were built to cross the Atlantic Ocean between Europe and the United States while cruise ships typically serve shorter routes with more stops along coastlines or among various islands.
For a long time cruise ships were never as large as the old ocean liners had been, but in the 1990s this changed and several new cruise ships in succession became the largest passenger ships ever built, superseding the records held by old Cunard liners such as the Queen Mary and the Queen Elizabeth.
The Queen Mary 2, which entered service in 2004, is of hybrid construction. It is marketed as an ocean liner as it is to dominate the transatlantic crossing market but with a profile more like a cruise ship than any previous liner aimed at that market. It supersedes the "Eagle Class" cruise ships of the Royal Caribbean line as the largest passenger ship ever built. However, it is not as fast as the Queen Elizabeth 2 (QE2) which it replaces on that route, and the QE2 was not as fast as the old ocean liners before it, such as the Queen Mary, Normandie, or SS United States.
