In the field of economics, international trade is heavily studied and analyzed. In theory, a world with no trade barriers (free trade) would make consumers and producers better off. It would allow for demand and supply to set prices based on which places can produce what goods most efficiently. More efficient production then leads to lower prices. In today's global market, free trade is virtually nonexistent. Instead, countries impose barriers to trade on each other that protect the markets and prices within their own country from being affected by cheaper imports being sold within its borders. This policy is called protectionism. There are different ways that protectionism works, which can be understood by the types of international trade barriers.
Import quotas set a particular quantity of a particular product that is allowed to enter into the country imposing the quota. For example, country A might create a quota that allows only so many cars to be imported to it from country B. Country A is trying to protect its own car-producing industry by trying to make sure that consumers have less access to the cars from country B. Overall, this raises the price of cars in country A, and reduces the amount of income for country B. Both country A and country B suffer in the long run.
Tariff is essentially another word for "tax." While there are several types of tariff barriers, they all essentially strive to do the same thing. They tax imports coming into a particular country. For example, country A puts a tax on imported bananas. The producers of bananas in other countries must all pay this tax to export their bananas to country A. The government of country A creates more revenue for itself by collecting the tax, and the price of imported bananas is higher for consumers in country A.
Other barriers to trade include regulations that require a certain percentage of domestically produced content or a certain level of quality for the product to be imported or exported. Some countries create lengthy and tedious requirements to get import permits that prevent countries from attempting to export to their country. These miscellaneous barriers carry the same purpose as the others: to protect an industry in one's own country that produces a certain good or service from outside competition that could result in job loss or revenue loss for that country. As countries try to protect their own domestic industries, they do a disservice to their consumers. Barriers to trade result in higher prices and lower revenue, overall, for both international and domestic producers.