This morning’s publication by the UK of its proposed tariff plan in the event of a hard Brexit has certainly got people talking and minds concentrated.

For years now, the "T" word has been central to commentary around the impact of the UK’s plans to leave the EU.

But do you know what tariffs are and why they are so important in the context of trade, particularly in our case the import and export of goods between Ireland and the UK?

Let’s look at the issue in more detail.

What is a tariff?

Put simply, a tariff or customs duty is a tax on imports.

There are generally three types.

Import duties are, as the name suggests, a tax on goods coming into a country and are most commonly used today.

Export duties, which are less in evidence in modern economies, taxed trade leaving a territory.

While transit duties are levied on a country through which a product travels on its way to another destination – again not so used today.

What is the purpose of the most commonly used import tariffs?

The purpose of such taxes is to protect domestic industries, companies and all important jobs.

By slapping a tariff onto certain goods coming into a territory, countries can make those imports more expensive.

That then hinders that competitor and gives something of a head-start or at least a level playing field to indigenous firms.

In particular, governments tend to put higher tariffs on the more vulnerable goods and sectors.

Another function of tariffs is that they can be used by governments to help their exporting firms.

This can be done by using the carrot of lower counter tariffs to force a reduction in the tax that third countries levy on exports being imported from that territory.

Tariffs can also be used to raise revenue for a state, but often this is a secondary consideration.

Who pays the tariff?

Tariffs are paid by the individual or organisation that is importing or exporting.

But ultimately in most cases it is the customer or consumer who will pick up the tab as it is generally passed onto them in the form of higher prices or charges.

That is why exporters here are so concerned about the imposition of tariffs as a result of Brexit, because if prices on their goods go up in the UK post-Brexit, they will be less competitive in that market and will suffer as a result.

Rates can vary very considerably.

For example, under the plans published today, the UK would hit poultry imports with taxes of up to €78.40 per 100kg, butter with a duty of up to €73.80 per 100kg, buses with a 16% tariff and certain seafood and fish products imports would rise by up to 24%.

Today’s list also gives some insight into the level of detail which customs tariffs go.

Take for example, the tyres and wheels category which includes "pneumatic tyres new of rubber of a kind used for buses or lorries with a load index of > 121".

While among the dozens of textile and clothing items listed is, "men’s or boys' bib and brace overalls of synthetic fibres industrial and occupational (excluding knitted or crocheted)".

How are tariff rates decided?

Each country has its own list of tariffs that apply to different goods, known as a schedule.

Typically they are set by governments as a percentage of a product’s customs value, although they can also be a specific duty on a weight or other measure.

Here, as we are members of the EU’s internal market, the state must apply the 'Common Customs Tariff' to imports of goods coming across the external borders of the EU.

The tariff is common to all EU members, but the rates of duty differ from one kind of import to another depending on what they are and where they come from.

The rates depend on the economic sensitivity of products, with no tariffs applied to some goods and heavy taxes on others.

The EU uses the tariff to ensure that domestic producers can compete fairly and equally on the internal market with manufacturers exporting from other countries outside the bloc.

Trade agreements are in place between the EU and a range of other territories, including New Zealand, Australia, Japan, Canada and Mexico which have led to more favourable tariff arrangements with those countries.

But if the UK leaves the EU on 29 March without a deal, there will be no trade agreement in place between it and Ireland.

In theory, rules set by the World Trade Organisation (a global body with 164 member governments) should then kick in.

However, what was published today by the UK represents a unilateral temporary modification of those rules which will last up to 12 months.

The question being raised by some experts though is, is it legal?

Comments welcome via Twitter to @willgoodbody