The Iron Maiden is a torture device which consists of a case large enough to hold a standing human. The door to the case can be closed and latched with the person inside, and a small hole at face level allows the torturer to interrogate the victim. The torture is provided by the numerous spikes inside the case and attached to the door of the case, which all pierce the victim when the door is shut. Because the spikes remain in the wounds, the victim bleeds very slowly and can last for days while inside. It was used heavily in the Middle Ages to punish religious heretics. It is well known today, as an example of the barbaric cruelty of medieval Europe.
Or at least, that's the story. In reality, it is believed that the Iron Maiden was never used during the Middle Ages, and in fact didn't even exist during that time. Its history was written largely by Johann Philipp Siebenkees, a German philosopher, in the 1700's. There has never been any evidence of an actual Iron Maiden that dates to earlier than the late 1800's. It's widely believed that the story of the Iron Maiden is a hoax, and although Iron Maidens do exist and may have been used from time to time since their popularization, they are not an authentic medieval torture device.
The Iron Maiden's grisly and dramatic nature has helped it to become an icon in popular culture. We are both attracted and repulsed by extreme torture, and this helps hoaxes which tap into our fears and fascinations to become widespread.