COP21 is actually the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change. 

That long winded title was created in Rio in 1992 where countries concerned about the impacts of climate change came together under the UN to do something about it. 

They signed a convention that entered into force in 1994 and has now been ratified by 195 countries, including the United States. 

The key aim is the "stabilisation of greenhouse gases in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system".

Essentially the world's governments have already committed to curbing human activities such as burning fossil fuels that release the gases that interfere with the climate. 

Hoorah you might think, problem solved. 

If only! 

The big problem comes when you try and get 195 different countries to agree on how to deal with the issue. Every year since then the Conference of the Parties has taken place with negotiators trying to put together a practical plan of action. 

This year's COP21 in Paris is the last chance for this process. Negotiators agreed back in 2011 that a deal had to be done by the end of 2015. They must now deliver that or the whole show will fall off the rails. 

Critics would say that the problem of climate change mustn't be that urgent if it takes 20 years to agree on a solution.

But defenders argue that it's taking such a long time because decisions are taken by consensus, meaning nothing is agreed until everything is agreed. The parties believe that despite this huge limitation, it is the best way of guaranteeing fairness. We all share the planet, they say, so all should have an equal say in what happens to it.