An indictment is a formal accusation of a felony, issued by a grand jury based upon a proposed charge, witnesses' testimony and other evidence presented by the public prosecutor (District Attorney). It is the grand jury's determination that there is enough evidence that the defendant committed the crime to justify having a trial voted by a grand jury. In order to issue an indictment, the grand jury doesn't make a determination of guilt, but only the probability that a crime was committed, that the accused person did it and that he/she should be tried. District Attorneys do not present a full case to the grand jury, but often only introduce key facts sufficient to show the probability that the accused committed a crime.
The Fifth Amendment to the U.S. Constitution provides that "No person shall be held to answer for a capital, or otherwise infamous crime, unless on presentment of a Grand Jury…." . Therefore grand juries are often used in charging federal crimes. However, states often use a "preliminary hearing" held by a lower court judge or other magistrate in place of a grand jury to determine whether the prosecutor has presented sufficient evidence that the accused has committed a felony. If the judge finds enough evidence that the accused committed a crime, the case will be ordered to be sent to the appropriate court for trial.