A cruise is a voyage taken for its own sake. The ship is as much the destination as the ports are, which is what separates it from a crossing, where the point is to arrive, and from a ferry, where the point is to carry people and vehicles over water that is in the way.
Two quite different things share the word. An ocean cruise sails between ports, usually returning to the one it left, with days at sea in between and a vessel large enough to be a resort in itself. A river cruise follows an inland waterway, ties up most nights, and keeps the bank in view throughout. The second is closer to a hotel that moves than to a ship that entertains.
The word also stretches across an extraordinary range of lengths, from a single night out of a home port and back to a circumnavigation lasting several months. Almost everything practical — what has to be arranged beforehand, what the days aboard are like, which season is the right one — follows from where a particular voyage sits on that scale.