One of Europe's top officials believes President Trump and his team need to get familiar with how the EU works.

"I do think that there is a room for explanations, because sometimes I have the impression that the new administration does not know the European Union in detail," said European Commission President Jean-Claude Juncker at a meeting of EU leaders. "But in Europe, details matter."

Several EU leaders have raised concerns over Trump's stance on the EU, NATO and trade agreements.

The EU is a complicated beast, a union that has grown from 6 to 28 members in six and half decades, and now encompasses 500 million people, a free trade area, single currency and customs union. EU countries have also signed up to common security and defense policy, and the majority -- but not all -- are members of NATO.

Even the biggest europhiles admit the EU can be difficult to understand. Here's a primer:

What is the EU?

The EU is an economic and political union of 28 European countries. Not all European countries are EU members. Some, such as Norway and Switzerland, have chosen to remain outside the bloc.

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Others, including Albania and Serbia, want to join but have not yet reached the level of economic development the EU requires. And some, like Russia or Belarus, have not been invited or expressed a desire to join.

So it's like the United States of Europe?

Not exactly. The EU was created in the aftermath of World War II. The idea was that countries that trade with one another and become economically interdependent are more likely to avoid conflict.

But the EU is not a federation, and although it has many common policies, each member country has its own taxation system, budget, parliament and government.

What about the U.K.?

The U.K. is still officially part of the bloc. Brits voted to leave the EU in a referendum last June, and Prime Minister Theresa May said she will start the formal withdrawal process by the end of March. Two years after that, Britain will cease to be a member.

Trump has made European leaders nervous by calling Brexit "a great thing."

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Is the EU the same thing as the eurozone?

No. Only 19 of the EU's 28 member states use the common currency, the euro.

The idea behind the common currency was to make it easier for companies to export and import goods and services across the bloc and to reduce the costs associated with volatile exchange rates.

But I thought the euro was a mess.

It has some major problems. Most stem from the fact that there are big differences between the individual economies of the eurozone, and there's no single eurozone government that can move money around between them. Some, like Greece and Italy, have borrowed a lot in the past and have had a hard time repaying their debts.

Others, like Germany, are pressuring them to implement painful reforms and cut spending. That is creating friction across Europe -- populist parties across the bloc are calling for the euro to be scrapped.

The European Central Bank has to strike a balance between the needs of all eurozone members -- not an easy task.

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This week, Trump adviser Peter Navarro even accused Germany of taking advantage of an undervalued euro and "exploiting" trade partners including the U.S. German Chancellor Angela Merkel rejected that notion.

So are there any borders in Europe?

Very few: 26 European countries, including Switzerland and Norway who are not members of the EU, agreed to take down their borders and introduce a free travel area.

However, some temporary border controls have been reintroduced recently after a wave of terror attacks in Europe.