What is the shutdown?
The law authorising funds to the keep the US government functioning expires at midnight tonight. If Congress fails to pass a new law, known in this case as a continuing resolution or a stopgap budget, to keep the money flowing, the federal government will have no choice but to begin putting hundreds of thousands of non-essential workers on unpaid leave and suspending services - or, to use the jargon, to shutdown.
So why doesn't Congress approve the new law?
Because the Republicans who control the House of Representatives have attached an amendment to the stopgap budget that would delay funding for President Obama's health reforms by a year. The amendment would also repeal a tax on medical devices that is meant to pay for the reforms, which are known as Obamacare. The Senate, which is controlled by President Obama's Democrats, has made it clear that it will not authorise any measure that includes changes to Obamacare.
What happens if they don't resolve their differences?
Federal government agencies will have no choice but to put hundreds of thousands of works - estimates suggest that the final tally could be around 800,000 - on temporary leave. At Nasa, for example, estimates suggest that over 90 per cent of the workforce will be asked to stay at home. At the Pentagon, active duty personnel, numbering around 1.4 million, will continue to work, but around 400,000 civilian employees might be put on unpaid leave. Moreover, many of those to who do continue working might see their paychecks delayed. Americas national parks and public museums would also close. And those outside the US thinking of apply for a visa would also face delays, depending on how long a shutdown lasts.