The GATT was the quasi-international organization that existed until 1995 to administer the trade agreement called the GATT 1947. The WTO was established in 1995. It provides the institutional framework for implementing the Uruguay Round agreements (including the GATT 1994), negotiating new trade rules and settling disputes between member countries.
The members of the WTO are nation states and the European Communities (European Union) as an organization. Each EU member state is also a member of the WTO. The European Commission speaks for all EU members at most WTO meetings.
A state can accede to the trade agreements as a new member after negotiations with its individual trading partners in the WTO and with the WTO as a whole. A protocol of accession memorializes the rights and obligations of the new members.