http://campus.udayton.edu/~grandjur/faq/faq1.htm
Good Grand Jury Explanation
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What is a grand jury?
A grand jury is a group of people that are selected and sworn in by a court, just like jurors that are chosen to serve on a trial jury (such as the jury in the O.J. Simpson criminal case or in the Louise Woodward ("au pair") murder case). In fact, the grand jurors are usually chosen from the same pool of people that provide trial jurors: A judge selects and swears in a grand jury, just like judges select and swear in trial juries. But grand juries differ from trial juries in several ways.
Lady Justice.jpg (28459 bytes) For one thing, grand juries may sit for longer. In the federal system, a grand jury can sit for up to 36 months, although it doesn't have to sit that long. The court that swears in a new grand jury can extend its term in 6-month increments, for a total of 36 months, but a federal grand jury may only sit for 18 months or so. State grand juries sit for varying terms: Depending on the state, a particular grand jury may sit for a month, six months, or even a year.
Unlike trial jurors, though, grand jurors don't convene every day. By the summer of 1998, for example, the D.C. grand jury that Independent Counsel Kenneth Starr was using to investigate the alleged relationship between President Clinton and former White House intern Monica Lewinsky was sitting two days a week. And even that is unusual; many federal grand juries sit only one day a week, and some may only sit twice a month. State practice varies, but a state grand jury might sit twice a month, or even only once a month. Sometimes, the grand jury doesn't convene unless a prosecutor asks it to, because the prosecutor has cases he or she wants the grand jury to hear.
Unlike trial juries, grand juries don't decide if someone is guilty of criminal charges that have been brought against them. Grand juries listen to evidence and decide if someone SHOULD be charged with a crime. In the O.J. Simpson case, the prosecutors were going to ask a grand jury to charge Simpson with murdering Nicole Brown Simpson and Ronald Goldman, but the defense attorneys persuaded the court that the grand jurors had heard too much about the case to be able to make an impartial decision. That is, the defense attorneys filed a motion saying the grand jurors were too prejudiced by what they had seen on television and read in the papers to be able to review the evidence against Mr. Simpson impartially, the way a trial juror should. The judge agreed with the defense attorneys, which is very unusual. Normally, defense attorneys fail when they try to claim that a grand jury is biased. Courts reject these claims on the theory that all the grand jury does is bring charges, so even if a grand jury is biased, the person they charge can still prove their innocence at trial. But a California judge bought the defense's argument in the case of O.J. Simpson (perhaps because of the extraordinary publicity surrounding Mr. Simpson) and, instead of trying to start over with a new grand jury, the prosecutors used another method to charge Simpson with the murders.