Alcatraz Island is one of the major tourist attractions of San Francisco.
The island is part of Golden Gate National Recreation Area.
The maximun security federal prison on the island once housed "famous" criminals like Al Capone and Robert Stroud, the Birdman of Alcatraz.
Tickets (round-trip by ferry and admission) for Alcatraz are available at the ticket booth at pier 41 (Fisherman's warf).
The ferry to the island departs from pier 41.
The first boat leave at 9:30 AM and the last boat leaves at 4:15 PM (summer months) or 2:15 PM (rest of the year).
Specially in the summer months tickets are sold out up to a week in advance.
It is strongly recommended to make reservations if you want to visit Alcatraz in the summer.
More information:
NPS - Golden Gate NRA - Alcatraz
The Blue & Gold Fleet: Alcatraz Tour Description and Schedule