A president is a person elected to run a board of administration, an association or even a country.
Most of the democracies are governed by a president who is therefore the head of the executive branch.
A chief executive officer is like the president of a big business corporation: he exercises full powers in his company.

Heads of state are not always known under the label of “president”. It's a matter of whether the democratic country is a parliamentary system or a presidential system.
In Great-Britain for instance, there is no president but a prime minister, who is head of the government and member of the parliament.
The president's scope of responsibilities varies from one country to another, so does the history of each country.

Most countries changed their monarchic system for a democratic system after the people couldn't stand the authority of a King, or a Queen any longer.
These changes cannot take place in peace, and for each country, the signs that herald the shift from one system to an other have similar contents:
France for instance beheaded Louis the XVI to start a democracy during what is known as “the age of enlightenments”: people were starving when a King was living grandly and a bourgeoisie started to think that it would be more advantageous to have an elected government on which to exercise pressure to decrease taxes. The first president of France, after the monarchy was reestablished, was Louis Napoleon Bonaparte, who betrayed his title of president to become a self-proclaimed emperor a few years after his election.

The United States has never been governed by anyone else than a president. Presidents' deeds are legitimate by the fact that they have been elected, directly or not, by a majority of people.
Their powers are great: in most democratic countries only the hand of the president can push the atomic button.

Some presidents are quite renown: Kennedy because he was murdered in the “prime”of his life, Barack Obama, who succeeded to George W Bush, because he is the first black president of the United States ever elected.
It's worth mentioning that some presidents have been made head of their state by illegitimate means. This is the case when a takeover happens.