What is a Grand Jury and why is it so controversial?

It would be very disappointing to find that the jury principal was fatally flawed, not the sure defence of the rights of the individual against the over-mighty power of the State that we have always believed.

A Grand Jury seems (for us) a superb idea. Following a significant and contested incident a random selection of 23 (?) citizens is summoned to hear the evidence, debate it and decide on a course of action. Rather than stilling protest, in the US the Grand Jury seems to foment it.

So could I ask our US contributors to explain (and I apologise for using this forum for FAQs).

Is a Grand Jury (GJ) as I’ve described it above?

What is wrong with the process that makes its verdicts so un-acceptable?