A monarch is a person who rules because one of their relatives ruled. A male monarch can be a king or emperor. A female monarch can be called a queen or empress.
This is monarchy, a system of ruling in which ruling starts by a member of a family, and continues in that family as a dynasty. Monarchs were very common in history. Many countries used to have a monarch, but no longer have them. Some countries that still have monarchs have other leaders that actually have the power.
Monarchs tended to have monarchs of lower honour below them, e.g. a Empire could include several kingdoms, and a kingdom would have several duchies or principalities, nations would rarely include all titles, usually going from duchy to kingdom instead of duchy to principality to kingdom. Prince/Princess means 'Ruler of a Principality' and 'son of king/queen'. Some other languages have separate words for them.
|Grand Principality||Grand Prince||Grand Princess|
|Archduchy||Arch Duke||Arch Duchess|
|Grand Duchy||Grand Duke||Grand Duchess|