The king is the one who lead a state (monarchy). He hasn't been elected but he gets the power for he is the descendant of a royalist descent.

Military chief, political decision maker and in charge of the economy, the king concentrated most of the powers in his hands. Of course, he had some ministers, but most of the time, they would serve as advisers more than executives. It is only when revolutions happened that monarchies had -for most- stopped existing or had let the place for democratic states or for republics.

Nowadays, the king has more a honorific role in a country's system, some sort of symbolic image that is exported across the frontiers. In those countries, there is no president but a prime minister and a government elected by the people (United Kingdom, Spain, Belgium...). Certain nations like France has always have a family of royal descent but they usually get some “ranks” more modest (count of Paris...).

The most ancient king of history would be Sargon, from the empire of Akkad (Mesopotamia), who reigned from 2334 to 2284 B.C. Louis XIV is the monarch who reigned for the greatest length of time in Europe: 72 years! But since he was 5 when he ascended the throne, he did not actually reigned for 12 years. The actual longest reign in the world is the one of Hirohito, Emperor of Japan for 62 effective years!