The European Union (EU) is a politico-economic union of 28 member states that are located primarily in Europe.
It means an internal single market through a standardized system of laws that apply in all member states.
Membership of the European Union has been a topic of debate in the United Kingdom since the country joined the European Economic Community (EEC, or "Common Market") in 1973.
So now they are voting to decide Britain’s fate with respect to the European Union and its the Remain campaign, backed by British Prime Minister David Cameron and Labour Party leader Jeremy Corbon versus the Leave camp, led by the former London mayor, MP Boris Johnson.
According to London-based political pollster Joe Twyman, it is an "extremely close" race.
"We know that over the last few polls, we've seen the gap between Leave and Remain narrow — sometimes one ahead, sometimes another — but now statistically speaking, it's a dead heat," he said.
"We expect that actually Remain could take it, but it's by no means outside the realms of possibility that Leave could manage it as well."