The grand jury is selected by the prosecutor with an eye toward obtaining a cross section of the community, or at least a body able to express community sentiment. A foreperson is elected to be in charge. At their hearings, only the prosecution, and not the defense, present evidence. It's mostly a rubber stamp for the prosecutor. At least 12 votes are needed to indict.
Grand juries have enormous powers.
They can consider hearsay evidence. They have investigatory powers, and can be used by prosecutors to investigate police corruption or scandals in city government. Every year, thousands of Americans are called to serve as jurors in a petit, or trial jury. It's the only work that citizens are expressly required to serve for their government. There are about 200,000 jury trials held every year, not that many given the full number of criminal cases.