An emperor is a male who rules an empire. An emperor usually comes to power when one of his parents, or relatives, dies. In some countries, people elected a new emperor from candidates.
The wife of an emperor is called an empress. A woman who comes to power in an empire is also called an empress, for example Catherine the Great.
The only emperor in the world today is the ruler of Japan. In history, many countries had emperors at different times, including China, Rome, Germany, Ethiopia, Turkey, India, France and Russia.