From electowiki

A caucus generally refers to, in the U.S., a process where voters physically convene to help give their favorite presidential candidate delegates in the race to become a major party nominee. The general rule is that voters are allowed to support whoever they like, and if a candidate doesn't have enough voters supporting them (enough 1st choices), then they are eliminated, and their voters may move to support other candidates, with a final tally being taken of the % of votes each candidate got in the caucus. This is somewhat imitated in the process of IRV. A common alternative, which many advocate as superior, is the primary election.