Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Non-tariff barriers to trade
One of the reasons why industrialized countries have moved from tariffs to NTBs is the fact that developed countries have sources of income other than tariffs. Historically, in the formation of nation-states, governments had to get funding. They received it through the introduction of tariffs. This explains the fact that most developing countries still rely on tariffs as a way to finance their spending. Developed countries can afford not to depend on tariffs, at the same time developing NTBs as a possible way of international trade regulation. The second reason for the transition to NTBs is that these tariffs can be used to support weak industries or compensation of industries, which have been affected negatively by the reduction of tariffs. The third reason for the popularity of NTBs is the ability of interest groups to influence the process in the absence of opportunities to obtain government support for the tariffs.