Merchant vessel

A merchant vessel or trading vessel is a boat or ship that transports cargo or carries passengers for hire. This does not include pleasure craft that do not carry passengers for hire. Warships are also not considered to be merchant ships.
Types.
They come in large number of sizes and shapes. They range from twenty-foot inflatable dive boats in Hawaii, to 5,000 passenger casino vessels on the Mississippi River. They include tugboats working in New York Harbor, to 1,000 foot oil tankers and container ships.
Some of the more common types are:
Fleets.
Most countries of the world operate fleets of merchant ships. However, due to the high costs of operations, today these fleets are in many cases sailing under the flags of other nations. These are countries that specialize in providing manpower and services at very good terms. Such flags are known as "flags of convenience". Currently, Liberia and Panama are two of the most popular flags of convenience. Ownership of the vessels can be by any country, however.
Today, Japan and Greece are two of the largest merchant fleets by capacity. In 2014, the two countries shipped about 30% of the world's tonnage. However, China is now the nation that owns the most merchant ships. 
During wars, merchant ships may be used as auxiliaries to the navies of their respective countries. They are called on to deliver military personnel and material.