A grand jury is a group of individuals who help a prosecutor decide whether to bring criminal charges, or indictment, against a suspect in a crime, according to Cornell University Law School Legal Information Institute. A petit jury is a group of citizens who try a question of fact.
Grand juries, usually composed of 23 people, may serve for months at a time and hear many cases, notes FindLaw. They work directly with a prosecutor and have a wide latitude to view evidence and interrogate witnesses. People who appear before a grand jury do not have attorneys present with them, and the court relaxes the usual rules of evidence. The grand jury does not decide guilt or innocence; it decides whether there is sufficient cause for the case to go to a regular criminal trial.
A petit jury, or trial jury, typically consists of six to 12 people and decides guilt or innocence based on the facts of the case, explains FindLaw. The jury remains seated for the entirety of the case, which can last for a few days or a few months. The procedure surrounding the proceedings is very strict, and jurors do not determine what evidence they see or question witnesses. Rather, attorneys enter evidence selectively, pursuant to rules and regulations designed to ensure that it is reliable.