yeah, I wiki'd it, too, when I realized I'd heard this term thrown around in crime dramas my whole life without any depiction of it, nor explanation. I'm still not really clear on it, though. Wikipedia said that it was a jury of 23 (give or take--there's a lot of innuendo when it comes to GJs) versus the "petit" jury of 12 for a trial. This implies that they are selected the same way, but I've never heard anyone selected for jury duty–including myself– who said it was for a grand jury.
Further reading repeatedly uses the word "retained" to describe them because, unlike the "single use" trial jury, they preside over all cases in the district for a given length of time, but it also implies that they are paid, though nothing is stated explicityly. In the "Criticisms" part, it discusses that they are frequently not a representative sample of the district and that there are no qualifications. This implies that they are appointed, but again, nothing is stated explicitly.
Sounds like an awesome gig. You join a shadowy organization, get paid, no qualifications, and apart from throwing scumbags in jail (well, determining if the state has a case, anyway,) you are required (at least in some districts) to inspect the jails and treasury and report any fraud you find i.e. whistleblow on abuse of power. Sign me up. I'm totally down.