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A kingdom is the most common type of monarchy. This means that the ruler or head of state of the country is a king. They get their position from inheritance. With some countries a female may become head of state as a queen.
When the current king or queen dies, then their eldest child becomes the new monarch.
For example, the United Kingdom has a king as head of state. His mother Queen Elizabeth II was, at the time of her father's death, his only child.
Not all countries have this system. Many countries today have no royal family. In their case, the head of state is a leading politician.