23 January 2013 Last updated at 15:59

What is the European Union?

The Prime Minister David Cameron has said people should be able to vote on whether the UK should be a member of the European Union.

But the public vote, which is called a referendum, will only happen if Mr Cameron's Conservative Party wins the next general election.

For 40 years in Britain, politicians have argued about whether it is better for the UK to be in or out of the EU.

But what exactly is the European Union? And how does it affect our lives?

What is the EU?

The European Union is a group of countries whose governments work together.

It grew out of the aftermath of the Second World War.

Many people felt that the best chance of preventing future wars, and of promoting peace and prosperity in Europe, was by countries working more closely 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.

Who is in the EU?

Who joined when?

• 1957 - Belgium, France, Netherlands, Germany, Italy and Luxembourg
• 1973 - Denmark, Ireland and the UK
• 1981 - Greece
• 1986 - Portugal and Spain
• 1995 - Austria, Finland and Sweden
• 2004 - Hungary, Poland, Czech Republic, Slovak Republic, Slovenia, Estonia, Latvia, Lithuania, Malta and Cyprus
• 2007 - Bulgaria and Romania

From a small group of nations in the 1950s, the European Union is now made up of 27 different countries.

The United Kingdom joined in 1973.

Croatia will become the 28th member when it joins on 1 July 2013.

Other countries interested in joining include Iceland, Serbia and Turkey.

The European Union currently has around 500 million citizens.

How do countries join the EU?

Countries have to prove certain things in order to be allowed to join:

1. They must show that they treat their people fairly, respect their human rights and allow them to vote in elections.

2. They must show that their economies are properly run. That means the government is sensible about the amount of money it spends and does not interfere too much in the way people do business.

3. Countries may have to make changes to their laws so that they don't clash with the laws of the EU.

European money - the euro

The euro is a type of money that is used by 17 of the 27 EU member states.

The notes and coins were introduced in January 2002, when 12 countries started to use them instead of their own currencies, like the French Franc and Italian Lira.

The eurozone, is what people call the European Union countries who use the euro.