The European Union – also known as the EU – is an economic and political partnership involving 27 European countries.
It began after World War Two to foster economic co-operation, with the idea that countries which trade together are more likely to avoid going to war with each other.
It has since grown to become a “single market” allowing goods and people to move around, basically as if the member states were one country.
It has its own currency, the euro, which is used by 19 of the member countries, its own parliament and it now sets rules in a wide range of areas – including on the environment, transport, consumer rights and even things such as mobile phone charges.
The 19 countries using the euro are: Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Portugal, Slovakia, Slovenia and Spain.