In Iowa if your in a Republican Caucus, you meet in your precinct with a precinct captain. Each candidate has a representative at each of them to make a presentation. At the end of the presentations, you write your choice of candidates on a slip of paper and turn it in. If the majority is for one candidate, that is who their caucas votes for.
If there is no clear majority, the candidates representatives lobby each of the other groups to try and get them to join their candidate until a clear majority elects someone.
If your at a Democratic Caucus, pretty much the same procedure occurs, but there is no paper vote. People get up and stand in the group that represents their candidate.
If your candidate falls below a certain percentage of the people in the room, your group is no longer counted.
I don't remember if they are dismissed, or allowed to join with another candidates group after being lobbied by the candidates representative.
That's how I remember it.
Traditionally only 15% of Iowans go to caucus meetings. This election cycle is showing that percentage may reach record highs.