What is Brexit?

On June 23, U.K. citizens will head to the polls to answer one question: "Should the United Kingdom remain a member of the European Union or leave the European Union?"

The decision to leave would rattle an establishment that has been in operation for decades. The EU is an economic and political partnership between 28 member nations committed to the prosperity of the region as a whole. Britain joined the European Economic Community, which would later be absorbed into the EU, in 1973 and has been a valuable contributing member since.

But in recent years, anti-European rumblings have become more pronounced in the country.

In 2013, in response to public outcries and increasing pressure from members of Parliament, Prime Minister David Cameron pledged to hold a referendum on the U.K.'s membership in the European Union if his Conservative Party won the next election. The party won, and Cameron made good on his commitment when the June 23 date was set in February.

Cameron laid out a plan in late 2015 that attempted to change the terms of the U.K.'s membership in the EU without leaving the group. His efforts fell short in a deal that was finalized with EU leaders in early 2016, and "Brexit" -- a splice of "Britain" and "exit" -- was born. The nation has since divided itself into the "remain" camp -- including Cameron and former Prime Minister Tony Blair -- and the "leave" camp -- including former London Mayor Boris Johnson.

The decision that faces U.K. citizens is a political, economic and emotional one, shrouded by uncertain outcomes and largely without precedence (the U.K. held a similar referendum vote in 1975, but 67 percent of citizens voted to stay). Here are a few charts that explain what is known about Brexit and the U.K.'s relationship with the EU.

[SEE: How the U.K. fares in Best Countries rankings.]

The EU represents 508 million people from 28 member states. The nations cumulatively comprise about a fourth of the world's economic output, totaling $18.5 billion in 2014, and the U.K.'s gross domestic product accounts for about 15 percent of the EU's total product.

Each member nation contributes a monthly sum to the EU's operating budget based on a tax formula in return for policies and programs that benefit the region. In pure budgetary terms, a number of countries that contribute most to the EU's operating budget stand to gain least from EU expenditures. In 2014, the U.K. gave nearly $5.5 billion more to the EU than it took from the budget, accounting for 8 percent of total national contributions, but less than 5 percent of the EU's total expenditures, according to the European Commission. Germany had an even greater negative balance -- more than $17 billion after contributing 18 percent of the budget. Italy had a negative balance of $5 billion after contributing 10 percent of the budget.

EU-funded initiatives in the U.K. highlighted by the European Commission include a $78 million investment in Superfast Cornwall, a commitment to connect at least 80 percent of homes and business in the rural county to "fibre-optic superfast broadband," and an $8 million investment in the Scottish Centre for Regenerative Medicine to fund the construction of new facilities for stem cell research.

The "free movement of people is one of the EU's fundamental principles," according to the EU freedom of movement and residence directive passed in 2004. Nearly 3 million people from other EU countries have migrated to the U.K., according to the United Nations Population Division, and 1.2 million from the U.K. have moved elsewhere in the Union. And while the U.K. remains a member of the EU, these 4 million migrants have the "right to move freely and live in another EU country," but there is no precedent as to what happens to those from a country or living in a country that removes itself from the EU.

In addition to people, goods, services and money are able to move freely within the EU's single market as if it were one country. Free trade within EU borders is supposed to mutually benefit all member nations and attract international partnerships to a large European trade bloc. U.S. President Barack Obama said that the U.K. would be at the "back of the queue" for a trade deal if it votes to leave the EU and that it could take a decade to negotiate a new agreement. The U.S. is currently the U.K.'s largest export destination.

Results of the referendum have been tough to predict. Many polls consistently report a split of less than 5 percentage points. Generally, though, the majority of younger people and richer people say they will vote "remain," while the majority of older people and poorer people say they will vote "leave."

The views of other EU citizens are much clearer: they want the U.K. to remain part of the club. " France is the only country where more than a quarter of the public says it would be positive for the EU if the UK departed," according to a Pew Research Center report from April. In an earlier survey of 28,000 European voters by Lord Michael Ashcroft, former deputy chairman of the Conservative Party who has become known for his political polls, only 10 percent said they would like the U.K. to leave the EU. Sixty percent of those surveyed said they would prefer the U.K. stay in the EU, and 30 percent said it didn't matter. At 24 percent, Luxembourg had the highest portion of people who said they would prefer the U.K. to leave the EU, and Lithuania had the highest portion of people that would prefer the U.K. to stay.

British and Irish citizens who live in the U.K. and are older than 18 can vote on the referendum, as long as they registered to vote before the June 9 deadline. A record number of 525,000 applications on June 6 caused the government registration website to crash and spurred an extension of the initial June 7 deadline. More than 1 million people registered to vote in the last days before the deadline.

Also eligible to vote are British and Irish citizens living abroad who have been registered to vote for at least 15 years and Commonwealth citizens living in the U.K.

If U.K. citizens vote "leave," transitioning out of the EU could effectively take years. But pressure on the voters is much more immediate; they'll have to make up their minds before heading to the polls this week.

Deidre McPhillips is a data reporter at U.S. News. You can find her on Twitter or email her at dmcphillips@usnews.com.