Tariffs

A tariff is a tax placed on imports. It is set per unit. For example the UK government could put a tariff of £5 per kilogram of beef imported into the UK. If the market price for beef in the UK without trade is £40, this is what the diagram would look like.

A - Represents the conversion of consumer surplus back to producer surplus (for domestic producers)
B - Gain in tariff revenue for the government (tariff per unit (£5) * Q2->Q4)

Shaded Area - Loss in welfare / deadweight loss

Extension in domestic supply after tariff is added is X->Y

Y->Z Represents the disequilibrium between domestic supply and demand after a tariff has been added. This means that the quantity of imports is Q2->Q4, filling the disequilibrium.