I've posted this before on the subject, but I particularly like the description given by Ken White at Popehat:

"Rather than tread over the ground well-described by my colleagues in the criminal defense bar, today I'd like to describe something else for you: what a federal grand jury proceeding looks like. From 1995 through 2000, I presented cases of varying complexity to federal grand juries as a federal prosecutor in Los Angeles. That experience did not inspire confidence in the process. Rather, it taught me that the adage that a grand jury will indict a ham sandwich is an understatement. A better description would be that the prosecution can show a grand jury a shit sandwich and they will indict it as ham without looking up from their newspapers. The notion that the Supreme Court relies upon — that the grand jury has a "historical role of protecting individuals from unjust persecution" — is not a polite fiction. A polite fiction would have some grounding in reality. It's an offensive fiction, an impudent fiction, a fiction that slaps you across the face and calls your mother a dirty bitch."

http://www.popehat.com/2014/02/27/the-kaley-forfeiture-decis...