The Grand Jury is an alternative to a Preliminary Hearing.
In a preliminary hearing the prosecution presents evidence to the judge in a trial-like proceding, in the presence of the defense. The purpose is not to determine guilt or innocense, but whether there is sufficient evidence for a full trial.
A Grand Jury is selected much like any other. The jury makes the decision about the validity of the charges. The procedure may be closed to the public. A prosecutor makes charges against various defendants or potential defendants, & subpoenas witnesses, mostly. Kenneth Starr had a Grand Jury. O.J. had a preliminary hearing.
They're just two ways of bringing a crime to trial.Neither determine guilt or innocense.