A grand jury is a type of legal body that is responsible for investigating and determining whether there is enough evidence to justify a formal trial. It typically consists of 16-23 citizens who are chosen at random from the community, and they hear testimony from witnesses and review evidence presented by prosecutors in order to determine if a crime has been committed and if it warrants further legal action. The grand jury's role is not to decide guilt or innocence but rather to assess whether there is enough evidence for the case to proceed to trial, which means they can either issue an indictment (if they believe there is sufficient evidence) or refuse to do so (meaning the case will likely be dropped). The grand jury system has been in place since the 12th century and serves as a way to ensure that only serious cases are brought before the court.