The Constitution guarantees that "the trial of all crimes ... shall be by jury." The jury system is one of the most fundamental tenets of the American judicial system. Thus, in all but the most minor cases and in impeachment trials, anyone accused in a civil or criminal case is ensured that a group of people, rather than an individual judge, will decide the outcome of the proceeding.
In the United States, there are two types of juries: grand juries and petit juries. The grand jury’s function is to listen to evidence provided by prosecutors against an individual and return a bill of indictment. If a grand jury indicts someone after they have heard the evidence presented by the prosecution, they are not saying that the person is guilty. They are simply saying that there is enough evidence to bring that person to trial. While grand juries in almost all cases follow the lead of the prosecutor, they are considered an important check on overzealous government prosecutors. In many states, however, the grand jury is rarely used, and has been replaced by a simpler system in which the prosecutor merely bring formal charges.
The petit jury is the jury that actually hears evidence in a trial. It is typically made up of 12 people. Some states have juries with fewer people. In most cases, jury verdicts need to be unanimous. In some other cases, however, particularly in civil cases, there must be some kind of extraordinary majority.