A form of government in which the head of goverment (a King or Queen) inherits the title.
Absolute Monarchy is where the King or Queen has absolute power and can do what he wants.
Constituitonal monarchy is when the King shares power with Parliament and must follow the rules of the constituition.
Originally, thousands of years ago, monarchy was so that one guy could control all and everyone would know who succeeded him. This way, there were no fights between people over things like laws or taxes because only one person decided that, and no fights over who would succeed him because it was his child. Unfortunately in many cases (I won't say most because that is an opinion) the ruler wasn't suited for the job and there were fights anyway. Because of this many countries either changed to Democracy
or changed to Constitutional Monarchy.
Absolute Monarchy: Louis XIV of France. Now obsolete.
Constitutional Monarchy: The current government of England.