The Monarch is an officer, representative or head of state, which may be hereditary or elected (as in some states as the ancient kingdom of Poland, which used a system of election or the current Malaysia which operates an alternation between monarchs inherited traditional nine states).
A nation or state headed by a monarch is called a monarchy. A system of government involving a monarch is a monarchy.
The term "king" means a male monarch, unless it is designated under another title as emperor where he runs an empire.
The term "queen" is applied to a female monarch (queen reigning), but also the wife of a king (queen who could be considered "consort"). A monarchy ruled by a king or queen is called a kingdom.
Nowadays, few monarchs exercising absolute power. Although many persist hereditary monarchies, the monarch acts in most of these countries, so defined by the constitution and has only symbolic power, real power is held by a parliament or legislature. For example, the United Kingdom maintains a hereditary monarchy. Currently, Queen Elizabeth II of the United Kingdom which is in power.
Read also Duke of York