Margaret Thatcher was Britain's prime minister (or 'PM') for almost 12 years between 1979 and 1990. She was the first woman ever to hold that job, or to lead any big country in Europe or America, and became famous the world over.
She was often called "the Iron Lady", and in fact was pleased when she was given the nickname - by a Russian newspaper - because she believed in being a strong leader.
When she became PM many people in Britian wondered whether a woman would be strong enough to do this very difficult job? She showed them the answer to that question was 'yes'. When women become leaders of countries, they are often compared to Margaret Thatcher, even when they are really very different people.