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Feeling a little lost on Brexit? Never really got your head around it in the first place? Let us walk you through it.

What is Brexit?

Brexit is short for "British exit" - and is the word people use to talk about the United Kingdom's decision to leave the European Union (EU).

What is the EU?

The EU is a political and economic union of 28 countries that trade with each other and allow citizens to move easily between the countries to live and work (click here if you want to see the full list).

The UK joined the EU, then known as the EEC (European Economic Community), in 1973.

Why is the UK leaving?

A public vote - called a referendum - was held on Thursday 23 June 2016 when voters were asked just one question - whether the UK should leave or remain in the European Union.

The Leave side won by nearly 52% to 48% - 17.4m votes to 16.1m - but the exit didn't happen straight away.

It was due to take place on 29 March 2019 - but the departure date has been delayed (we will explain in more detail below).

What has happened so far?

The 2016 vote was just the start. Since then, negotiations have been taking place between the UK and the other EU countries.

The discussions have been mainly over the "divorce" deal, which sets out exactly how the UK leaves - not what will happen afterwards.

This deal is known as the Withdrawal Agreement.

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What does the withdrawal agreement say?

The withdrawal agreement covers some of these key points:

How much money the UK will have to pay the EU in order to break the partnership - that's about £39bn

What will happen to UK citizens living elsewhere in the EU and, equally, what will happen to EU citizens living in the UK

How to avoid the return of a physical border between Northern Ireland and the Republic of Ireland when it becomes the frontier between the UK and the EU

Media playback is unsupported on your device Media caption Adam Fleming summarises the Brexit agreement in one minute(ish)

It also includes a transition period, which has been agreed to allow the UK and EU to agree a trade deal and to give businesses the time to adjust.

That means that if the withdrawal agreement gets the green light, there will be no huge changes between the date of Brexit and 31 December 2020.

Another, much shorter, document has also been drawn up that gives an overview of what the UK and EU's future relationship will be in the longer term.

This is the political declaration. However, neither side has to stick exactly to what it says - it is a set of ambitions for future talks.

The deal was agreed by the UK and the EU in November 2018, but it also has to be approved by British MPs.

Have MPs backed the withdrawal agreement?

Well, no. They have voted against it three times.

On 15 January they rejected the deal by 432 votes to 202 - a record defeat.

Then on 12 March, after Theresa May - the prime minister at the time - had gone back to the EU to secure further legal assurances, they rejected it again.

And on 29 March - the original day that the UK was due to leave the EU - MPs rejected it for a third time (this vote was slightly different as it did not include the political declaration).

Is that why the UK didn't leave on 29 March as planned?

Yes. As MPs did not approve Mrs May's withdrawal deal, she was forced to ask other EU leaders to delay Brexit.

The deadline was delayed until 31 October - but, unable to see a way forward, Mrs May stepped down as PM and was replaced by Boris Johnson.

Why do people oppose Mrs May's deal?

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There is a broad range of complaints, many of which claim the deal fails to give back to the UK control of its own affairs from the EU.

One of the biggest sticking points has been over what happens at the Irish border.

Both the EU and UK want to avoid the return of guard posts and checks (here's why), so something called the backstop - a sort of safety net - was included in the deal.

What is the backstop?

The backstop is meant to be a last resort to keep an open border on the island of Ireland - whatever happens in the Brexit negotiations.

It would mean that Northern Ireland, but not the rest of the UK, would still follow some EU rules on things such as food products.

Theresa May insisted that, if all went as planned, it would never be used.

But it has annoyed some MPs, who are angry that the UK would not be able to end it without the EU's permission and so EU rules could remain in place for good.

Other MPs would prefer the UK to stay closer to the EU - or even still in it.

And others say Northern Ireland should not be treated separately from the rest of the UK.

So, what happens now?

Boris Johnson has said he wants to renegotiate the withdrawal agreement with the EU - and remove the backstop.

If he succeeds in doing this, it will still have to be approved by MPs before being passed into UK law.

However, EU leaders have consistently said they will not renegotiate the withdrawal agreement and that the backstop is an essential part of any deal.

What happens if there is no new deal?

If the prime minister fails to convince the EU to change the withdrawal agreement, he has promised to take the UK out of the EU without a deal on 31 October.

Leaving the EU without a withdrawal agreement would be known as a "no-deal Brexit".

Image copyright PA Media Image caption New PM Boris Johnson has promised to take the UK out of the EU with or without a deal

What happens if the UK leaves without a deal?

"No deal" means the UK will have failed to agree a withdrawal agreement.

It would mean there would be no transition period after the UK leaves and EU laws would stop applying to the UK immediately (more on that here).

The government says it is preparing for this potential situation.

It expects some food prices could rise and customs checks at borders could cost businesses billions of pounds. (Read the government's report here).

It has published a series of guides - which cover everything from mobile roaming on holiday to the impact on electricity supplies.

Here is a list of 10 ways you could be affected by a no-deal Brexit.

Is it possible Brexit will not happen?

It is still written into law that the UK will be leaving, even though the deadline has shifted.

The European Court of Justice has said the UK could cancel Brexit altogether without the agreement of other nations - but few politicians publicly back this option.