Both presidents and kings are heads of state, but they reach their positions in very different ways. Numerous types of government can be presided over by a president, ranging from constitutional democracies to dictatorships. Kings can also work within a variety of political systems, including parliamentary governments and absolute monarchies. The differences between the two political leaders are important, and are sometimes used to highlight differences between governments as well.
A king is a head of state who inherits his position from his family. The female counterpart of a king is a queen. He is part of a monarchy which may stretch back for many generations. The king is ruler for life unless he abdicates, and is usually revered as the sovereign leader of his nation. In some countries, the king acts as an absolute ruler over his people, in an absolute monarchy. In other nations, the monarch is more like a figurehead, and political decisions are made by elected and appointed officials such as ministers and members of parliament. In nations which have retained their sovereigns, the king and his relatives are called the royal family, and special honors are accorded to them.
A president is an official who is elected, either directly by the people or through a representative system such as the Electoral College. The president usually has a set time limit on his or her term, and some nations also limit the number of terms which a president can occupy. As head of state, he or she participates in the running of the government, and usually has veto power over bills proposed by the legislature. The president also appoints cabinet officials.
In most cases, a president is also associated with a democratic system of government in which all citizens may actively participate in their nation's politics. The president is one of many elected officials who work together to lead the country with the input of the people. A king is also perfectly capable of serving in this position, but when most people think of kings, they think of an absolute monarchy.
In some cases, a dictator may take the title of president. This designation is usually technically incorrect, since most dictators intend to rule for life, and often pass the position on to children or favored people in their political parties. Since a “president” in this context has no term limits, and is usually not associated with a democratic system of government, most nations do not recognize dictators who call themselves “presidents.”