First: What even is the EU? The quick-and-dirty way to think about it is as a sort of USA-like, except instead of states, it's a union of nations. The 28 member countries agree to open their borders to other EU members, share a common market, and abide by various social and political policies. This is slightly different from the Eurozone, which created a common currency for 19 of the 28 member countries. So it’s really more like USA-but-not-as-much-and-also-way-more-complex. Opinion polls leading up to Thursdays referendum have been nail-bitingly close — and may have big economic repercussions. Last week, when one poll showed voters leaning toward Brexit, the Pound Sterling briefly tanked.