Twenty seven nations in Europe came together for their common benefit to form the European Community Law. That became the basis for the formation of the EU Law. The laws of this body functions along side the individual laws of each member nation. When there is a conflict between the member nation's law and the EU law, the EU law takes precedence. On the surface it seems as if that would present a problem within the member states, but it does not. The reason this super-national law is able to work is based on the treatise each member signed when they joined the union.
The EU treaties are the primary source of Eu law. These treaties give power to the body. Policies are set forth through these treaties. Institutions that have legislative power are set in place by these treaties, therefore they can enact necessary laws and regulations. There are two forms of legislative acts of this body, and they are regulations, and directives. Regulations become law immediately once passed by the legislative body, and override domestic law of each member nation. Directives give a desired results from the body, but leaves the implementation of details to the individual member states. The EU Court of Justice is the institution that insures uniformity in the application, and interpretation of the laws.