Cruise ships are large passenger ships used mainly for vacationing. Unlike ocean liners, which are used for transport, they typically embark on round-trip voyages to various ports-of-call, where passengers may go on tours known as "shore excursions". On "cruises to nowhere" or "nowhere voyages", cruise ships make two- to three-night round trips without visiting any ports of call. As of 2021, the world's largest passenger ship is Royal Caribbean's Symphony of the Seas.