A monarchy is a system of government in which one person reigns. The ruler of a monarchy, known as a monarch, is usually a king or queen, but may hold a different title such as emperor or grand duke. The monarch is often only the head of state, not the head of government. Most modern monarchies, such as Britain and Denmark, are actually governed by parliaments, and are termed "Constitutional Monarchies."
Monarchies are typically hereditary -- that is, a child of the monarch becomes the new monarch when the monarch dies, usually going to the oldest male child. The exact details of who inherits the position are called succession. The rules for succession, such as what happens if the monarch's eldest child is a female or under age, or if the monarch dies childless, are often very complex, and vary from country to country.