The words duty and tariff both refer to taxes due to the government on imported goods. The two words can often be used interchangeably, but occasionally only one is correct, as in the following examples:
“How much duty did you have to pay?”
“The minister spoke about the economic effect of tariffs.”
As these examples show, when we talk about the system of government-imposed duties or the list of such duties, the word tariff is more appropriate, but we use duty or customs duty when we mention specific amounts.