In a monarchy an emperor and king are both rulers, but the power associated with them is different. They can be compared to regional manager and the CEO of a company. An empire can have many kingdoms within it; the emperor rules the entire empire while kings (or queens) rule smaller kingdoms within the empire. While the king (like the regional manager) has total control over his territory, the emperor (like the CEO) is the one who makes the final decision for the entire region.
This distinction is not universal. There are many examples in history where large kingdoms have been called empires but have been ruled by a single monarch, a king or queen. For example, King George V, Edward VIII and George VI were all kings of the United Kingdom and emperors of India.
What is a Kingdom?
A kingdom is a state or province, which may be independent or a part of a bigger empire.
Empire and emperor
When a king conquered his neighboring kingdoms and united them all under his command, he became the emperor of his entire region, which was now called an empire. So an Emperor has control over a vast area and King’s power is limited.
An emperor had the highest power in the whole empire. He represented the empire as a whole which was further divided into further kingdoms. Most kings were engaged in battle during those times to expand their kingdom. Kings usually represented a larger system which had and customs regarding succession, duties, and powers. The Emperor was the one who incorporated the rules and policies of the empire. The king within the empire helped in implementing these rules.
Source of power
Traditionally, hereditary succession within members of one family has been the most common mode of electing a King while an emperor can be elected either by inheritance within the family or by defeating a King.