The General Agreement on Tariffs and Trade (GATT) is an agreement between states and other entities on the rules for trade.
It was signed in Geneva, Switzerland in 1947 by 23 states. It was not an international organization, although there had been from the beginnings plans to establish an International Trade Organization. The agreement was amended several times, including most recently by the Marrakesh Agreement in 1995, which ended the Uruguay Round of trade negotiations, which established the World Trade Organization.
In 1962 the Kennedy Round instituted the "Trade Expansion Act of 1962." This act gave greater power to the executive branches of the respective nations. The Executive was granted negotiation powers up to 50%.
GATT functions as the foundation of the WTO trading system, and remains in force, although the 1995 Agreement contains an updated version of it to replace the original 1947 one.
The GATT, as an international agreement, is very similar to a treaty. Under United States law it is classed as a congressional-executive agreement. It is based on the "unconditional most favored nation principle." This means that the conditions applied to the most favored trading nation (i.e. the one with the least restrictions) apply to all trading nations.