The grand jury is composed of approximately 14-23 persons selected from the community. These people meet in secret and hear evidence from the prosecution to determine if there is probable cause that a crime was committed and that the defendant committed the offense. The grand jury meets in secret; no judge is present, no defense attorney is present. The prosecutor calls and questions witnesses; based on this one-sided presentation, the grand jury makes its decision. The standard is very low.
Only a simple majority of the grand jurors need to find probable cause for an indictment (the formal charging document) to be issued. Grand juries can and are used by the prosecutors to investigate cases where no one has been arrested as well as cases where someone has already been arrested. Grand juries can issue subpoenas and compel persons to testify and submit documents to them. If you are subpoenaed to appear as a grand jury witness or to produce documents to a grand jury, you should seriously consider seeking competent legal representation. Preliminary examination
A preliminary examination is a hearing where a magistrate judge hears evidence and makes a determination as to whether there is probable cause that the defendant committed the offense charged. However, preliminary examinations almost never occur in federal court because the case is indicted before a grand jury prior to the PE date. Misdemeanors do not require a preliminary examination and this information does not apply to them. The vast majority of cases brought in federal court are felony charges.