An emperor is a male who rules an empire. The word is taken from the Latin language imperator. Often it is capitalized.
A woman who comes to power in an empire is called an empress. The wife of an emperor is also called empress. An emperor or empress is often a hereditary monarch and comes to power when one of his parents, or relatives, dies. In some countries, people elected a new emperor from candidates.
The only emperor in the world today is the Emperor of Japan (tennō), but he lacks political power. The true leaders of Japan are the Diet and Prime Minister because the country is a parliamentary constitutional monarchy.
The English word comes from Latin, the language of the old Roman Empire. At first, an imperator was a powerful general (army leader) but, after Augustus, it was only used by their most powerful rulers. There have been many countries in history whose leaders are called "emperor" in English. The leaders of the Byzantine Empire (basileus, autokrator) in Greece, the Holy Roman Empire (imperator, Kaiser) in Austria and Germany, Russian Empire, Serbia and Bulgaria (tsar), and the Ottoman Empire (sultan, padishah, khagan, kaysar) in Turkey all said they were just new parts of the old Roman Empire.
The leaders of other countries who said they ruled the whole world or called themselves "King of Kings" are frequently also called emperors in English. For example, the old rulers of China (Huangdi), Iran (Shah) and Ethiopia (Negusa negast) are all known as emperors in English. There have also been emperors of France, Brazil and Mexico, and the rulers of the United Kingdom called themselves the Emperors and empresses of India for a while.