A grand jury decides whether there is probable cause to charge. It is not a trial, and the differences matter to anyone who receives its process.
Composition and secrecy. A body of citizens sitting over an extended period, in secret. Secrecy rules bind jurors, prosecutors and court personnel, but not witnesses, who are free to describe their own testimony.
One-sided. The prosecutor presents; there is no defence case, no judge in the room, and the rules of evidence largely do not apply. Hearsay may be presented. The target has no right to appear, though prosecutors sometimes permit it.
Counsel may not enter. A witness may consult counsel outside the room, and the practical rhythm of a grand jury appearance involves stepping out after questions.
Subpoenas come in two forms: for testimony and for documents. Both are enforceable by contempt. Objections — relevance, burden, privilege, the act of production doctrine — are raised by motion to quash before the return date.
The Fifth Amendment protects an individual from compelled self-incrimination but not an entity, and a custodian may generally be compelled to produce corporate records even if they incriminate the custodian personally.
Immunity. Use immunity compels testimony while barring its derivative use; transactional immunity is broader and rarer at the federal level.