The WTO has about 150 members, accounting for about 95% of world trade. Around 30 others are negotiating membership.
Decisions are made by the entire membership. This is typically by consensus. A majority vote is also possible but it has never been used in the WTO, and was extremely rare under the WTO’s predecessor, GATT. The WTO’s agreements have been ratified in all members’ parliaments.
The WTO’s top level decision-making body is the Ministerial Conference which meets at least once every two years.
Below this is the General Council (normally ambassadors and heads of delegation in Geneva, but sometimes officials sent from members’ capitals) which meets several times a year in the Geneva headquarters. The General Council also meets as the Trade Policy Review Body and the Dispute Settlement Body.
At the next level, the Goods Council, Services Council and Intellectual Property (TRIPS) Council report to the General Council.