The caucus system in the State of Iowa is quite complex, but only if you are a Democrat. The Republicans (and in this process one must be registered as a Republican or Democrat) simply show up, vote for their preferred candidate, the votes are counted and that candidate becomes the Republicans choice for the Iowa delegates that are chosen to attend the national Republican Convention, probably in August or September (the national vote is in November).
The Democrats have an uneccessarily complex system that's difficult to explain here... suffice it to say that the end result is the same... a candidate of choice to be backed by the Iowa Democrats at the National Convention.
Many voters in Iowa are disenfranchised... minortiy party voters cannot caucus... but that's the way Iowans want it. Almost all of the following primaries, such as the next in New Hampshire are more straight forward and less cumbersome.
By the way, very few voters in Iowa actually participate, but, again that's a cherished way of doing it that they aren't lkely to change...