A grand jury is composed of 23 members. Ordinarily, they listen to evidence presented only by prosecutors and determine whether there is probable cause to believe that a crime has been committed by the individuals or organizations under investigation. If the grand jury finds there is probable cause, it issues a written statement of the charges that is called an indictment. Grand juries meet in secret, which protects the reputations of those under investigation. A grand jury's work becomes public only if an indictment is issued. After that, the accused will be brought before a judge to be arraigned. The accused can then decide to plead guilty, or plead not guilty and go to trial.
A petit jury -- also referred to as a trial jury -- consists of six to 12 members. In criminal trials, they listen to evidence presented by both prosecutors and the defense. They determine whether the accused is guilty beyond a reasonable doubt -- a higher standard of proof than the probable cause finding made by the grand jury. In civil trials, petit jurors listen to evidence presented by both sides and find for the plaintiff or defendant. Both criminal and civil trials are held in open court, except in rare instances when the judge seals a portion of the proceedings.