A Grand Jury is part of the system of checks and balances, preventing a case from going to trial on a prosecutor's bare word.  It, as an impartial panel of ordinary citizens, must first decide whether there exists reasonable cause or probable cause to believe that a crime has been committed.  The Grand Jury can compel witnesses to testify before them.  Unlike the trial itself, the Grand Jury's proceedings are secret; the defendant and his or her counsel are generally not present for other witnesses' testimony.  The Grand Jury's decision is either a "true bill," there is a case to answer, or a "no true bill."  Since their deliberations were done secretly, a constable was assigned to serve the Grand Jury and assure that their activities were legal.  Grand Jurors typically are drawn from the same pool of citizens as a petit jury, and participate for a specific time period. 

Source:  Wikipedia

The Traverse Jury or Petit Jury is a trial jury impaneled to hear evidence in a civil or criminal case.  This type of jury is a sworn body of persons convened to render a rational, impartial verdict and a finding of fact on a legal question officially submitted to them, or to set a penalty or judgment in a jury trial of a court of law.

 Source: Wikipedia

In the early circuit court records for Van Buren County, TN, the sheriff would issue a venire facias (writ to summon qualified jurors to court) at the outset of the term of court.  From the jury pool, a group of twelve men (remember, these were 19th century trials) were selected to be on the Grand Jury.  Other remaining men in the jury pool were used for the Petit or Travers Jury.  For example:  (p. 63 of 1852 Van Buren County, TN minutes) "The balance of jurors who do not compose the Grand Jury are retained to serve as Traverse Jurors."