On the news you might hear about "grand jury investigations" and not know exactly what it entails. Right now in Texas, Gov. Rick Perry is going through a grand jury investigation over his alledged corruption.
In the state of Florida, a grand jury is an investigating/reporting/accusing agency of the circuit court. It can also be of the Florida Supreme Court in the case of the statewide grand jury. It's made up of citizens of a certain number who have been summoned by a judge of the circuit court. It is answerable to no one and no agency of the government, except for the court that selects the group of people who make up the jury.
It isn't like most criminal and civil hearings. Just about everything done in a grand jury is kept secret. All the sessions, deliberations, and voting are done in private. If you have been summoned in to appear before a grand jury, you must have legal representation. As soon as you receive the target letter for a grand jury investigation, you must call an attorney ASAP. The purpose of grand jury proceedings is to determine whether or not there is enough evidence to charge a person with a crime, and also to determine what charges should be brought. The grand jury doesn't decide if the person/company is actually guilty of the crime they are accused of.
Perhaps the most important thing to remember is to not ignore the letters from the federal government. If you do that you will most likely be indicted.