Simply put, the customs value is the value of an item at the point when it crosses the border into a certain customs territory. It consists of the invoice value and any deductions and additions that may apply. Under internationally recognized regulations, a six-stage calculation method has been introduced as part of the GATT Customs Valuation Code (CVC). The provisions of the CVC have been implemented in the Customs Code (CC) of the European Union.

Source: Wikipedia, Zoll online, edited (only available in German)