A cruise ship or cruise liner is a passenger ship used for pleasure voyages, where the voyage itself and the ship's amenities are part of the experience. Cruising has become a major part of the tourism industry, with millions of passengers each year. The industry's rapid growth has seen nine or more newly built ships catering to a North American clientele added every year since 2001, as well as others servicing European clientele. Smaller markets such as the Asia-Pacific region are generally serviced by older tonnage displaced by new ships introduced into the high growth areas.

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.