The customs union was one of the EU's earliest achievements. It abolished customs duties at EU countries' national borders and put in place a uniform system for taxing imports.
Customs officers are now found mostly at the EU's external borders. They not only keep trade flowing, but perform a wide range of tasks to protect Europeans.
The customs union is a single trading area where all goods circulate freely, whether made within the EU or imported from outside. A Finnish mobile phone can be dispatched to Hungary without paying any duty and without any customs control.
Duty on goods from outside the EU – say a TV from South Korea – is paid when they first enter the EU, but after that there is nothing more to pay and no more checks.

Sample of counterfeit products.
Despite this, customs activity in the EU remains vital, especially given the sheer volume of goods entering the bloc. EU customs services handle nearly 20% of total world imports – over 2 billion tonnes of goods a year. To do so, they process well over 200 million declarations annually.
The list is a long one:

Another major duty of customs in the EU is to tackle fraud, in particular 3 types:
The EU is constantly working on updating and automating procedures which will smooth trade across its internal and external borders.
Within the next couple of years, businesses will be able to throw away all their paper customs forms and the customs networks of all EU countries will be fully integrated electronically, providing a one-stop-shop for traders to sort out customs dealings throughout the EU.
EU customs officers do a vital job in collecting statistics. Their records contribute to decisions on whether to introduce limits on goods which may be competing unfairly with EU products. The trade flow data they collect also helps European policymakers detect economic trends.