What is an Iowa caucus?

It's a very blizzardy Jan 32rd here in New Jersey during Winter Storm Jonas, and I have time to do all sorts of things, like click on the links in The New York Times.  I found this link to a video of an Iowa caucus.  It was a Des Moines caucus in 2008--I don't know if this more than one.  In this video, the Democrats gathered in a large gym.  They counted themselves, and then divided up by candidate.  Candidates with less than 15% of the crowd were considered "non-viable" and then their supporters could scatter to other candidates.  It was quite a process of people engaging in the political process.  One woman was trying to convince some men to support Obama.  One man with a Hilary sign got hysterical, shouting that Obama didn't raise his hand to salute the flag. Another man said to the woman, "Could you give him a Valium?"  It is a fascinating look at what will be happening in Iowa, as Bernie Sanders and Hilary Clinton go toe-to-toe.