What is European Union and what countries belongs to it?

As detailed in the official UE website, the European Union is:

"The European Union is a unique economic and political union between 27 EU countries that together cover much of the continent." 

The countries of the European Union share common rules in agriculture, transport, fishing, industry, etc.

This means that:

• No customs duties are paid on goods shipping between EU member states.
• All countries apply a common customs tariff for imported goods outside the EU.
• Goods imported legally can circulate within the EU without more customs controls.

The countries belonging to the EU are:  

*UK is in process of getting out of EU

It must be taken into account that the Canary Islands, Ceuta, and Melilla have different tax rules so the shipments with this origin or destination are subject to the payment of tariffs and customs taxes.

Gibraltar and Isle of Man are also subject to tariff payments.