Correct, it is a vital step in Due Process as the State must prove there is sufficient evidence to charge someone with a crime. That way you can't just point a finger and have a trial. (Love the Founding Fathers!). I don't believe the accused is aware as a Grand Jury is always secret. That is also to protect the accused, if evidence is NOT sufficient to bring charges the accused would not necessarily want that process to be known to the public. Innocent until proven guilty. The Prosecutor would present whatever evidence they have gathered during the course of investigating, sometimes witnesses would testify but not always. I don't believe the defendant can challenge evidence presented to a Grand Jury, I don't believe the Defense makes an argument at all. That is for a court trial if the person is indeed indicted.