Usually the size and the purpose. The period of time sometimes distinguishes the name. Modern navy combat ships are generally divided into seven main categories. The categories are: Aircraft Carriers, Battleships, Cruisers, Destroyers, Frigates, Submarines, and Amphibious assault ships. There are also support and auxiliary ships, including the minesweeper, patrol boat, and tender. During the age of sail, the ship categories were divided into the ship of the line, frigate, and sloop-of-war. Frigate is a name which has been used for several distinct types of warships at different times. It has referred to a variety of ship roles and sizes. From the 18th century, it referred to a ship smaller and faster than a ship-of-the-line, used for patrolling and escort work rather than fighting fleet actions. In modern military terminology, the definition of a frigate is a warship intended to protect other warships and merchant marine ships and as anti-submarine warfare (ASW) combata...