Alcatraz island sits in the middle of the Bay, about 20 minutes by boat from the piers. It served as a penitentiary from 1915, until Robert Kennedy, as Attorney General, ordered it closed in 1963. The island, and it's elderly buildings, is now the responsibility of the National Parks Service and they operate the various trips for tourists.
It is an enormously popular tourist attraction, and tickets sell out days in advance. I think I managed to get one (online) because I wanted just a single, and chose the Night Tour. I think, and I'm writing this a week later, it may be the very best tourist visit I have been on.
It is a cold and forbidding place, and at night even more so; we had an evening of contrast, strong sun as we stood in line on shore, but the temperature dropping quickly as the boat moved out into the water and the fog came rolling in. The deep sirens of ships in the bay, and the booming foghorn on the Golden Gate Bridge, add to the eeriness. It takes just twenty minutes to cross to the island.
There were perhaps two hundred of us on the tour, and we were all remarkably quiet during the ascent from the small dock, up the hill to the main prison buildings. The guides are all from the National Park Service, and this undoubtedly contributes to the quality of the tour: they are very knowledgable, take their jobs very seriously, although not without charm and humour.
For much of the tour within the buildings you wear headphones and listen to a commentary which guides you from place to place. The story is all told by ex prisoners and ex guards, and this adds enormous authority to the commentary: no embellishment, due regard for the facts, and respect for the way prisoners were treated in "an earlier age".
In an era when most similar attractions are presented with audio, plasma screens, and other interactive devices, this was incredibly simple. The buildings stand largely as they were when the prison closed, nothing is fake or scenic; the audio does the rest.
If you are ever going to be in San Francisco, I'd urge you to book in advance, and go prepared to walk some distance, on uneven ground, and wrap up warm.