from Wikipedia Brussels:
Brussels (French: Bruxelles, pronounced [bʁysɛl]; Dutch: Brussel, pronounced [ˈbrɵsəɫ]) is the largest city in Belgium, and the administrative heart of the European Union (EU). The City of Brussels in the Brussels-Capital Region is the country's capital. Brussels has grown from a 10th century fortress town founded by Charlemagne's grandson into a city of over one million inhabitants.
Brussels is also capital of the Brussels-Capital Region, of Flanders and of the French Community of Belgium. It is not, however, the capital of the Walloon Region (Wallonia), whose capital is Namur.
Depending on the context, the word Brussels may mean the largest municipality of the Brussels-Capital Region officially called the City of Brussels (ca. 140,000 inhabitants), the Brussels-Capital Region (1,067,162 inhabitants as of 1 February 2008) or, the metropolitan area of Brussels (ca. 1,350,000 inhabitants).
Brussels is often considered the de facto capital of the European Union, and hosts key EU institutions such as the Commission, Parliament and the Council. Hence, many other pan-European organisations are also headquartered in the city. NATO is also based in Brussels.