Iowa Caucus

Iowa (I-owa, not I-o-wa) is a state West of Wisconsin/Illinois, South of Minneapolis. Sort of centre-middle of the mainland US.
They have a system where Iowa is divided into precincts (1,682), and at each of those precincts, caucuses (groups of people) meet - the republicans together and the democrats together (but in a different place) and they vote (in different ways, of course) for who they want to be their delegate. They're voting not for a president, but for delegates in the county conventions, who then decide on the delegates for the congressional district state conventions, who then decide on the delegates to go to the national convention, who then vote for the  candidates in the presidential elections.
Sounds weird and complicated? Apparently there is low voter turn out in Iowa because of poor understanding of the process!
I think this is how most voting stuff is decided in Iowa, but it becomes very important in the presidential campaigns, because it is the first official event in the nominating process. And of course, there is the usual obsession on statistics - on how accurate the vote is according to who eventually gets to the presidential election.

Only voters who are registered with either major party take part in this one.
Just to clarify, we are in the first round voting - that is, both main parties are fielding multiple candidates, and registered party members in each state votes for who they would like to go through to the second round of voting - the actual vote for the president.
Republicans just vote with either a show of hands, or a secret ballot.
Democrats actually physically stand or sit in groups that represent the support for each candidate. After a headcount, the candidate group with the least amount of supporters is dissolved, and those people go and stand in other groups, until one candidate has the most supporters. sort of like preferential voting, except without the ballot paper.
I think actually each state has a certain amount of delegates that go to the final convention and vote for the final candidate. Iowa is a very small state, but it is, as mentioned before, the first official event in the presidential nominating process. People like to theorize about how the results in Iowa reflect (or don't reflect) the mood across America.
More information? http://2016iowacaucus.com/how-iowa-caucus-works/