Customs, tariff, duty, and tax are four words that are often used interchangeably, because they are all types of monetary charges or fees that are incurred. However, each word has a bit of a difference between them, regarding what they are placed on and how they are enforced.
The first term, customs, is a word that is often used regarding charges that are incurred upon entry into a country. This is in reference to items that are bought in one country and carried over into the next. Customs is enforced by the law, and is a varying set of charges that are put on specific goods, and not on the individuals who are carrying the goods.
A tariff is a charge that is placed upon imports, and sometimes exports, from one country to the next. Often tariffs are used to regulate trade or goods being brought into a country.
There are different types of tariffs that can be levied. A revenue tariff is a charge that is placed on a good that cannot be produced in a country; this is to set up steady revenue. A specific tariff is a general charge that is placed on all goods.
A duty is another charge that can be incurred; a duty can be put on items that are produced in a country or items that are imported and sold in the country. These are important in helping create revenue for a nation.
And a tax is a regulated charge that has been put on services, goods, and even property. It is used as a way to secure the goods and services, and also helps to generate revenue.