What's 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's the EU? The EU is a political and economic union of 28 countries which trade with each other and allow citizens to move easily between the countries to live and work. 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.
What's 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.
What does the withdrawal agreement say? The withdrawal agreement covers some of these key points:
(1) How much money the UK will have to pay the EU in order to break the partnership - that's about £39bn.
(2) What will happen to UK citizens living elsewhere in the EU, and equally, what will happen to EU citizens living in the UK.
(3) 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. A length of time, called the transition period, 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.
Have MPs backed the Withdrawal Agreement? No, they've 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 had gone back to the EU to secure further legal assurances, it was rejected again. And, on 29 March, the original day that the UK was due to leave the EU, MPs rejected it for a third time.
Is that why the UK didn't leave on 29 March as planned? Yes. As MPs did not approve Theresa May's withdrawal deal, the prime minister was forced to ask other EU leaders to delay Brexit. The new deadline is 31 October. However, the UK can leave before then if the prime minister can somehow get her deal approved by Parliament.
So what happens now? If MPs had passed the deal, there would have been one more step - putting the deal into UK law with the Withdrawal Agreement Bill being passed by MPs in Parliament. But now the prime minister is putting the Bill to Parliament anyway - using it as a kind of alternative way to allow MPs to vote on the deal to try to get it into law. It hasn't yet been published so we don't know exactly what it will say, but the PM has said it contains new guarantees on workers' rights, environmental protections and the Northern Irish border. Mrs May also said MPs will get a vote on whether to hold another referendum - but only if they back her deal in Parliament. It is expected to come to Parliament in the first week of June. It could be thrown out almost straightaway on its "second reading". If that happens, Mrs May is expected to resign. But if it gets through that it will go on to the next stage where it is gone through line by line and changes can be introduced. It could contain wording which would get rid of the need for another "yes/no" vote on the deal.
And then...? If the Bill passes, Brexit could happen before the 31 October deadline. If the Bill fails, then the way forward is totally unclear - though senior politicians have warned it would lead to either leaving the EU without a deal, or Brexit being cancelled entirely. The government had been talking with Labour to try to come up with a way of getting enough votes for the Bill to pass. But those talks have now ended without agreement.
Why do people oppose the deal? There are 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, 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. The prime minister insists that if all goes as planned it will 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. On 11 March, Mrs May and the EU released a statement, giving added legal reassurances that the backstop plan, if it ever needed to be used, would only be temporary. Mrs May hoped the statement would persuade her MPs to vote for her deal, but it was still rejected.
So could Brexit actually not happen at all? 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 politically, that's not likely to happen.
What happens if the UK leaves without a deal? "No deal" means the UK would have failed to agree a withdrawal agreement. That would mean there would be no transition period after the UK leaves, and EU laws would stop applying to the UK immediately. The government says it is preparing for this potential situation. It expects some food prices could rise and checks at customs could cost businesses billions of pounds.