The 20th Century was the period between the 19th Century and the 21st Century. It's history now though all adults lived through at least part of the 20th Century. Early in the 20th Century women got the vote in many Democratic countries.
The 20th Century saw World War I and World War II. It saw the rise and fall of Hitler, of Nazism and of Nazi Germany, also the formation of Israel. The 20th Century saw the rise and fall of the USSR and of Communism in Europe. The 20th Century saw the formation and expansion of the European Union.
The 20th Century saw any number of scientific and technical developments. There was radio, television, Computers and the start of the Internet. There were also Nuclear weapons and the threat of a Third world war between the Americans and the Soviets.