In most seafaring countries, the merchant marine (or merchant navy) is a fleet of ships used for commerce that sometimes complements the navy. These fleets may be divided into several categories:

• Freighters, or cargo ships, which today are mainly container ships.
• Coasters, smaller ships which are normally not ocean-going.
• Tankers for the transport of fluids, such as oil, gas and chemicals.
• Cruise ships, which travel to beautiful parts of the world and are effectively floating holiday hotels. The cruise trade developed after airplanes took over from ships the transport of passengers all over the world.

In the United States, the term usually refers to the U.S. Merchant Marine, which serves as an auxiliary to the U.S Navy in times of war.