When traveling abroad, the international motor insurance card, also known as the Green Card, serves as proof of an existing motor liability insurance policy in the bearer’s home country. The card does not need to be carried within the EU, since here, the license plate is considered the sole proof of insurance (see License plate agreement). However, carrying the card is recommended in view of the formalities necessary in the case of an accident. The card is mandatory when traveling to the following countries: Albania, Bosnia and Herzegovina, Iran, Iraq, Israel, Morocco, Macedonia, Moldova, Tunisia, Turkey, Ukraine, and others. The Green Card is issued by motor liability insurance carriers.
Source: EU-Info Deutschland, Wikipedia, edited (onyl available in German)