Some questions and answers about the Iowa caucuses this Thursday:

Q: What is a caucus?

A: A party meeting at the precinct level at which U.S. citizens express their candidate preferences and pick delegates to their county conventions. It is the lowest level of party politics -- the real grassroots. These meetings, held in each of the state's nearly 1,800 precincts, typically draw anywhere from a handful of people in rural areas to hundreds in suburban areas.

Q: Who takes part?

A: Any U.S. citizen who resides in Iowa and who is old enough to vote in the November general election and is a member of the party is eligible. Traditionally, only a small number of Iowans show up. This year, about 120,000 to 150,000 people are expected to vote in the Democratic caucuses, while 80,000 to 90,000 are likely to participate in the Republican contest.

Q: Why is it politically significant?

A: Persuading a group of average citizens to show up in support of a candidate is considered a sign of organizational strength. Each candidate courts politicians and activists at the state and local level in hopes of getting strong numbers of supporters to show up and participate. At the same time, the caucus system allows candidates to develop and hone their message before relatively small groups.

Q: What happens at a caucus?

A: Participants, led by a chairman or chairwoman, indicate their preferences for their party's presidential nomination, pick delegates to their county conventions and discuss party business, including their party platforms.

Q: What happens next?

A: Delegates chosen at the caucuses go to the county convention later in the year. There, the field is winnowed and delegates are chosen for the district convention. This happens again at district meetings and again at the state convention, where delegates are named to attend the party's national convention.

Q: Why are the numbers different?

A: The Republicans essentially hold a straw poll -- a head count -- at their precinct caucuses, reporting real numbers. One head, one vote.

The Democrats do not report straight numbers, but use a mathematical formula to determine support for a presidential candidate in percentages. A candidate must have the support of 15 percent of those present at any meeting, precinct caucuses through the state convention, to remain "viable." This is meant to ensure greater consistency throughout the process.

Q: Will there be exit polls in Iowa?

A: Yes. The Associated Press and the television networks will survey voters as they enter the caucus sites. Those surveys will help readers understand what issues and qualities motivated Iowans to vote for a specific candidate.

Q: How did the Iowa caucuses get started?

A: A commission appointed after the riots disrupted the 1968 Democratic National Convention recommended proportionate representation and affirmative action. Iowa Democrats decided to use new rules in 1972, adopting a regulation that there must be a month between events -- the caucuses, county, district, state and national conventions. The caucuses wound up being held as early as January.