With some help from the graphics department at USA Today, here's a short explainer.
Iowa has been the opening act for presidential politics since 1972 and state leaders have fought hard to keep it that way because it's a gold mine.
Every four years, the media circus comes to town with money to spend on everything from hotels to car rentals to restaurants.
A Caucus is different from the more common primary elections used by most other states.
There are no voting booths, instead a caucus is a local political party meeting where they discuss and vote for their favorite candidate and discuss the party's platform.
The two major parties caucus differently.
Republicans simply vote via secret ballot or a raised hand.
Democrats divide into groups based on which candidate they support, but during the caucus they can try and convince one another to change sides.
It's one of the reasons that a Caucus can take up to two hours.
A candidate doesn't have to win Iowa to become president, but no candidate who's finished worse than third has ever gone on to win the White House.
It's a chance to get some momentum for the primary season which will conclude when both Republicans and Democrats choose a nominee at their respective party conventions in July.
Photos:2016 presidential candidates
