What are caucuses? How to they differ from primaries?
Party caucuses are a form of election chosen by certain states instead of the traditional primary election. In a party caucus, voters meet in a private home or public building and vote by standing in a designated part of the room. In this stage, there may be more than two candidates running for the nomination of the party, so there may be multiple groups forming as voters show their decision. if a candidate does not have at least 15% of the voters there, the candidate is no longer "viable" and is no longer in the running in that state. Voters supporting other candidates can debate and persuade the non-viable candidate's supporters to join them before the final vote is taken. The candidate with the highest number of voters wins the caucus and receives delegates to the convention in proportion to the vote received.
Both primaries and caucuses are elections used by the party to determine who will be the party's nominee in the general election. Primary elections allow for a straight vote for one candidate only, although some states allow for cross-party voting (an "open" primary"), while others require voters to vote for the candidates of their own party (a "closed" primary). The Democrats allocated their delegates according to the percentage of votes each candidate received in the primary or caucus. The Republicans arranged their delegates in a manner similar to the Electoral College, where the winner in that state received all the convention delegates. That is why McCain clinched the Republican nomination sooner and easier than Obama did for the Democrats. He had to fend of an insurgent campaign by Hillary Clinton, who almost received as many delegates as Obama did. Obama reached the threshold of 2118 delegates in early June, 2008.
A breakdown of Democratic delegates can be found at Realpolitics.com along with current polling data.