A grand jury is a legal body empowered to conduct official proceedings and investigate potential criminal conduct, and determine whether criminal charges should be brought. A grand jury may compel the production of documents and compel sworn testimony of witnesses to appear before it. A grand jury is separate from the courts, which do not preside over its functioning.
The United States and Liberia are the only countries that retain grand juries, even though some other common law jurisdictions formerly employed them, and most others employ some other form of preliminary hearing. Grand juries perform both accusatory and investigatory functions. Investigatory functions of grand juries include obtaining and reviewing documents and other evidence, and hearing sworn testimonies of witnesses who appear before it. Grand juries' accusatory function is to determine whether there is probable cause to believe that one or more persons committed a certain offence within the venue of district court.
A grand jury in the United States is usually composed of 16 to 23 citizens, though in Virginia it is composed of fewer numbers for regular or special grand juries. In Ireland, they also functioned as local government authorities. In Japan, Law of July 12, 1948 created the Kensatsu Shinsakai (Prosecutorial Review Commission or PRC system), inspired by the American system.
The grand jury is so named because traditionally it has greater number of jurors than trial jury, called a petit jury (from the French word petit meaning "small"). Wikipedia