Cruise ships operate mostly on routes that return passengers to their originating port. In contrast, ocean liners do "line voyages" and typically transport passengers from one point to another, rather than on round trips. Some liners also engage in longer trips which may not lead back to the same port for many months.

Traditionally, an ocean liner will be built to a higher standard than a cruise ship, including stronger plating to withstand ocean voyages, most commonly crossing the North Atlantic. The only traditional ocean liners in operation, as of August 2008, are Queen Mary 2 and Queen Elizabeth II (to be retired in November 2008), both part of the Cunard fleet. The liners RMS Queen Mary and SS United States are also extant, with long-standing plans to return the latter to service, although this appears increasingly unlikely given its age and condition.

Early years

The first vessel built exclusively for this purpose was the Prinzessin Victoria Luise, commissioned by Albert Ballin, general manager of Hamburg-America Line. The ship was completed in 1900.

The practice of cruising grew gradually out of the transatlantic crossing tradition, which never took less than four days. In the competition for passengers, ocean liners added many luxuries — the Titanic being the most famous example — such as fine dining and well-appointed staterooms.

In the late 19th century, Albert Ballin, director of the Hamburg-America Line, was the first to send his transatlantic ships out on long southern cruises during the worst of the winter season of the North Atlantic. Other companies followed suit. Some of them built specialized ships designed for easy transformation between summer crossings and winter cruising.

Jet age

With the advent of large passenger jet aircraft in the 1960s, intercontinental travellers largely switched from ships to planes, sending the ocean liner trade into a slow decline. Ocean liner services aimed at passengers ceased in 1986, with the notable exception of transatlantic crossings operated by the Cunard Line, catering to the niche market who enjoy the few days of luxury and enforced idleness that a liner voyage affords. In comparison to liner crossings, cruising voyages gained popularity; slowly at first but at an increased rate from the 1980s onwards. Initially the fledgling industry was serviced primarily by small redundant liners, and even the first purpose built cruise ships were small. This changed after the success of the SS Norway (originally the ocean liner SS France, which was converted to a cruise ship) as the Caribbean's first "super-ship". Since then the size of cruise ships has risen dramatically to become the largest passenger ships ever built.Fact: date=September 2008