Definition
A customs union means that the participating states adopt a unified set of tariffs with regard to goods coming in from outside the free-trade area. Without this, the free-trade area could result in each type of good being imported into the state with the lowest tariff and then reexported (tariff-free) to the other states in the free-trade area. This would be inefficient. The Treaty of Rome committed the original six states in the Common Market to creating a customs union by 1969. A customs union creates free and open trade within its member states, bringing great economic benefits. Because of these benefits the customs union remains the heart of the EU and the one aspect widely copied elsewhere in the world.
EC-Turkey Association (Customs Union)
The Customs Union Between Turkey and the European Union
EU - Turkey Customs Union - Questions and Answers