A cruise ship is a passenger ship used for pleasure voyages when the voyage itself, the ship's amenities, and sometimes the different destinations along the way (i.e., ports of call), form part of the passengers' experience. Transportation is not the only purpose of cruising, particularly on cruises that return passengers to their originating port (known as "closed-loop cruises"). On "cruises to nowhere" or "nowhere voyages", cruise ships make 2-to-3 night round trips without any ports of call.In contrast, dedicated transport-oriented ocean liners do "line voyages" and typically transport passengers from one point to another, rather than on round trips. Traditionally, shipping lines build liners for the transoceanic trade to a higher standard than that of a typical cruise ship, including higher freeboard and stronger plating to withstand rough seas and adverse conditions encountered in the open ocean, such as the North Atlantic. Ocean liners also usually have larger capacities for fuel, food, and other stores for consumption on long voyages, compared to dedicated cruise-ships, but few ocean liners remain in existence - note the preserved liners and Queen Mary 2, which makes scheduled North Atlantic voyages.
Although often luxurious, ocean liners had characteristics that made them unsuitable for cruising, such as high fuel-consumption, deep draughts that prevented their entering shallow ports, enclosed weatherproof decks inappropriate for tropical weather, and cabins designed to maximize passenger numbers rather than comfort (such as a high proportion of windowless suites). The gradual evolution of passenger-ship design from ocean liners to cruise ships has seen passenger cabins shifted from inside the hull to the superstructure and provided with private verandas. Modern cruise ships, while sacrificing some qualities of seaworthiness, have added amenities to cater to water tourists, and recent vessels have been described as "balcony-laden floating condominiums".

The distinction between ocean liners and cruise ships has blurred, particularly with respect to deployment, although differences in construction remain. Larger cruise-ships have also engaged in longer trips, such as transoceanic voyages which may not return to the same port for months (longer round-trips). Some former ocean-liners operate as cruise ships, such as Marco Polo, although this number is diminishing. The only dedicated transatlantic ocean liner in operation as a liner As of December 2013 is Queen Mary 2 of the Cunard Line. She also has the amenities of contemporary cruise ships and sees significant service on cruisesCruising has become a major part of the tourism industry, accounting for U.S.$29.4 billion, with over 19 million passengers carried worldwide as of 2011.. 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 ships. These are displaced by new ships in the high-growth areas.As of 2019 the world's largest cruise-ship was Royal Caribbean International's Symphony of the Seas.