Container shipping is different from conventional shipping because it uses 'containers' of various standard sizes - 20 foot (6.09 m), 40 foot (12.18 m) , 45 foot (13.7 m), 48 foot (14.6 m), and 53 foot (16.15 m) - to load, transport, and unload goods. As a result, containers can be moved seamlessly between ships, trucks and trains. The two most important, and most commonly used sizes today, are the 20-foot and 40-foot lengths. The 20-foot container, referred to as a Twenty-foot Equivalent Unit (TEU) became the industry standard reference so now cargo volume and vessel capacity are commonly measured in TEU. The 40-foot length container - literally 2 TEU - became known as the Forty-foot Equivalent Unit (FEU) and is the most frequently used container today.
The container sizes need to be standardized so that the containers can be most efficiently stacked - literally, one on top of the other - and so that ships, trains, trucks and cranes at the ports can be specially fitted or built to a single size specification. This standardization now applies across the global industry, thanks to the work of the International Organization for Standardization (ISO) that in 1961, set standard sizes for all containers.
Proper loading or "stuffing" of containers is very important to the safety and stability of the containers and the ships, trucks and trains that transport the containers. In 2008, the World Shipping Council (WSC), together with the International Chamber of Shipping (ICS), published Transport of Containers by Sea - Industry Guidance for Shippers and Container Stuffers to aid those loading containers.
In 2010, WSC and ICS issued a joint statement calling on the International Maritime Organization (IMO) to establish an international legal requirement that all loaded containers be weighed at the marine port facility before they are stowed aboard a vessel for export.
Containers are generally constructed of aluminum or steel with each container size and type built according to the same ISO specifications, regardless of where the container is manufactured.
Shipping containers are available in a variety of types in addition to the standard dry cargo container often referred to as "special" equipment. These special containers include open end, open side, open top, half-height, flat rack, refrigerated (known as "reefer"), liquid bulk (tank), and modular all built to same exterior lengths and widths as the standard dry cargo containers. There are more than 17 million container units equating to more than 27 million TEU in the global container fleet.
Open tops are used for easy loading of cargo such as logs, machinery and odd sized goods. Flat racks can be used for boats, vehicles, machinery or industrial equipment. Open sides may be used for vegetables such as onions and potatoes. Tank containers transport many types of liquids such as chemicals, wine and vegetable oil.
Every container has its own unique unit number, often called a box number that can be used by ship captains, crews, coastguards, dock supervisors, customs officers and warehouse managers to identify who owns the container, who is using the container to ship goods and even track the container's whereabouts anywhere in the world.