
Stolen from wikipedia:-
There are three similar units of mass called the ton:
1. long ton (simply ton in countries such as the United Kingdom which formerly used the Imperial system of weights and measures) is a weight ton or gross ton, and is 2,240 pounds (exactly 1,016.0469088 kg).
2. short ton (usually called simply ton in North America or sometimes called a net ton) = 2,000 lb (about 907.18474 kg).
* Harbour ton used in South Africa in the 20th century, equal to 2000 pounds or one short ton.
3. The metric ton, or tonne, is 1,000 kg (or 1 Mg) or approximately 2,204.6 pounds.