Alcatraz is like a really big and old building in the middle of the ocean. It used to be a prison a long time ago. When bad guys did something very very bad, they were sent to Alcatraz to be locked up and kept away from everyone else.
The prison was on Alcatraz Island which is in the San Francisco Bay, and the only way to get there was by boat. It was really hard to get out of the prison because the water around the island was very cold, the currents were really strong, and often there were big waves. So the bad guys who were sent there had a really tough time trying to escape.
The prison was also famous for having some of the worst criminals in the country like Al Capone and "Birdman" Robert Stroud. They were kept in small cells that were like tiny rooms and they were only let out for a short time each day to do some exercise.
After many years, the prison was closed because it was too old and too expensive to keep maintaining. Now, people can visit the island and take tours of the old prison to see how it used to be.