This refers to the various organisations, institutions, treaties and traditional relations making up the European area within which members work together on problems of shared interest.
An essential part of this architecture was established by the Treaty on European Union, which formed three pillars: the European Community (first pillar), the common foreign and security policy (second pillar) and cooperation in the fields of justice and home affairs (third pillar). Matters falling within the second and third pillars are handled by the Community institutions (the European Council, the Council, the Commission, the European Parliament etc.), but intergovernmental procedures apply.
Source: Treaties Office Database