From Tara:
The Des Moines Register has a great little graphic that will explain just how caucusing works here in Iowa. As you'll see, there are some significant differences between the Republican and Democratic rules.
The main difference, as far as I am concerned, is that Republicans do a paper ballot vote and Democrats do a "walking caucus," moving around the room to gather in "preference groups" for their candidate of choice. If the preference group doesn't represent a large enough percentage of the total number of attendees, that candidate is not "viable" and those supporters must realign into other preference groups. The Republican paper ballot does not include a second "realignment" ballot; the percentages are simply reported as they are tallied on the initial vote count. With so many candidates in play this year, that will probably result in the winner having under 25 percent of the total votes cast.
The goal, at least at the Democratic caucuses, is to send forward delegates to the District and State conventions--delegates who have agreed to support the candidate their "preference group" chose. In 2008, for example, Barack Obama got over 37 percent of the delegates, and Hillary Clinton and John Edwards each got just under 30 percent of the delegates.
By the time of the State convention, the caucus and primary race has usually winnowed down the field to a single candidate, and departing candidates have urged their delegates to support the eventual nominee. In 2008, Hillary Clinton left the race quite late (early June), just a short while before the State convention later that month. On her departure from the race, she encouraged her delegates to move to Obama, which most of them then did.
I have chaired my precinct caucus twice--in 2004 and 2008--and have served as a delegate several times. This year, I plan to attend, but as our entire county's Democratic organization will be doing a combined caucus, (you probably know that we already have our nominee for President all picked out and there won't be round after round of "realigning" as in other years), I won't be holding the microphone and "herding kittens."