The European Union is a group of countries whose governments work together. It's a bit like a club. To join you have to agree to follow the rules and in return you get certain benefits. Each country has to pay money to be a member. They mostly do this through taxes. The EU uses the money to change the way people live and do business in Europe. Countries join because they think that they will benefit from the changes the EU makes.
All the places in Europe Belgium 1957 France 1957 Netherlands 1957 Germany 1957 Italy 1957 Luxembourg 1957 Denmark 1973 Ireland 1973 UK 1973 Greece 1981 Portugal 1986 Spain 1986 Austria 1995 Finland 1995 Sweden 1995 Hungary 2004 Poland 2004 Czech Republic 2004 Slovak Republic 2004 Slovenia 2004 Estonia 2004 Latvia 2004 Lithuania 2004 Malta 2004 Cyprus 2004 Bulgaria 2007 Romania 2007 In January 2007, the European Union had 494 million citizens.
A lot of what the EU does is about bringing people in Europe closer together. It tries to make it easier for Europeans to buy and sell things to each other. This is done by changing the rules that control trade.
Click here for a list of the Union's aims
It is also getting rid of controls that stop people from EU countries moving around freely inside the Union.
The changes have not always been popular in the UK.
The EU backs things like exchange visits that help ordinary Europeans to understand what they all have in common.
But there are some who think that the EU does too much, mainly if it tries to create one rule for everyone in Europe and then makes people obey it even if they dislike that rule.