Those Countries decided to work together to achieve better economic integration, under the assumption that the growth of international economic linkages would have been paralleled by wealth growth as well as an enduring, even everlasting, period of peace after the two World Wars that almost destroyed the whole european continent.
A distinguishing factor of the European Union is the abolition of customs and the creation of a common market where goods, services and people could move freely. The citizens of the European Union in fact, can move and trade freely within the member countries. Another distinguisching factor is that at the economic level, the EU has various policies including not only trade and competition, but also agriculture, energy, regional development and social welfare, among the others.
In some fields the EU is similar to a federation of states (such as the U.S.); on the other hand, on a large number of policies it looks closer to a typical international organization like ONU or WTO.
In this European Union wiki, we will explain how the european institutions work, how the member states cooperate within the European Union framework, and how decisions are taken and implemented on a various number of policies.