NATO (North Atlantic Treaty Organization) is a military alliance of 30 countries, primarily in Europe and North America, established after World War II to provide collective defence against potential threats. 

The member countries of NATO agree to mutual defence provisions, which means that if one member country is attacked, all other member countries will come to its aid. 

The European Union (EU) is a political and economic union of 27 European countries established after World War II to promote cooperation and integration among its member states. The EU has a single market where goods, services, people, and capital can move freely. 

It also has a common currency (the Euro) and a standard set of policies and regulations in trade, agriculture, and the environment. The EU is not a military alliance, but many of its member countries are also members of NATO. In summary, NATO is a military alliance focused on collective defence, while the EU is a political and economic union that concentrates on cooperation and integration.