King vs Emperor
To understand the difference between a King and an Emperor, one must first know the difference between a kingdom and an empire. Both King and Emperor are titles that refer to the ruler of a particular area, but depending on the area where their reign is, the title differs. The word king refers to a male sovereign. It is important to know that the male sovereign is the hereditary ruler of an independent state or a Kingdom. An emperor, on the other hand, refers to the sovereign of an empire. This is the main difference between the two words, king and emperor. It is interesting to note that a sovereign in the rank of an emperor is higher than the sovereign in the rank of a king. Both kings and emperors were considered as Gods within their kingdom or empire by their subjects. They were very much respected. Kings and emperors were always interested in expanding the borders of their kingdoms and empires.
Who is a King?
The word King is a title that refers to a male sovereign. It is important to know that the male sovereign is the hereditary ruler of an independent state or a kingdom. However, a king can be the ruler of only a part of a much larger empire. In that case, it only shows that a king is a ruler of any independent state or a dependent region. The female ruler in a kingdom is known as the Queen. This queen can be the wife or the mother of the king. Also, the queen can simply be the ruler of a kingdom where there is no king.

King James II and VII
Who is an Emperor?
On the other hand, an emperor is a ruler of an entire empire. There can be many vassal kings in that empire. These vassal kings are in charge of the various small kingdoms in the big empire. These rulers had the title of kings, but they definitely had lesser powers than the emperor.
It is the emperor who manages and looks after all the affairs of an entire empire. On the other hand, a king is bestowed with limited responsibilities by the emperor. These responsibilities and duties pertain only to the limited piece of land to which he is declared as a king.

Wilhelm II, German Emperor and the King of Prussia
It is known from history that many vassal kings have paid taxes to the most important emperor, in the form of land taxes collected from the individual members of the society. The emperor, on the whole, takes care of all the small kingdoms managed by individual kings.
The female ruler in an empire is known as the Empress. An empress can be the mother or the wife of the emperor. The empress can simply be ruler of the empire.
What is the difference between King and Emperor?
• The word king usually is a title, and it refers to a male sovereign. It is important to know that the male sovereign is the hereditary ruler of a kingdom. An emperor, on the other hand, refers to the sovereign of an empire, which may comprise many small kingdoms.
• A king can be an independent ruler of an independent state or a vassal king or a dependent king of a dependent state. The king becomes a dependent ruler only when his kingdom belongs to a larger empire. An emperor is always an independent ruler.
• An emperor looks after all the affairs of an entire empire as a whole. A king under the emperor may only have limited responsibilities as the emperor desires. An independent king has the same powers as an emperor as there is no one superior to him.
• The female ruler in a kingdom is the Queen. The female ruler in an empire is the Empress.
• An emperor comes into power through conquest or inheritance. A king also comes into power through conquest and inheritance. The king can also come to power through election like in the ancient Kingdom of Rome.
• Examples for emperors are the Japanese and Chinese emperors and Napoleon Bonaparte. Examples for kings are King George V and George VI.
These are the differences between the two words, namely, king and emperor.
Images Courtesy: King James II and VII and Wilhelm II, German Emperor and the King of Prussia via Wikicommons (Public Domain)
This is a very poor and bad definition. The title emperor and Kaiser both come from the Roman empire, who were the original empire, and any who takes the title claims to be a successor to it. “Emperor” comes from the latin word “imperetor” meaning commandor, and Kaiser comes from Cæsar, the last name of Julius and Augustus Cæsar whose name came to become synonymous with imperial authority.
We often translate oriental words for exalted rulers such as the Japanese “Mikado” as emperor because it have words for lesser rulers like “daimyo” that we like to translate as king despite it making more sense to call them dukes. But ultimately it is just an attempt to standardise different traditions.
And to define an empire as “any monarchy who have a sovereign that likes to stile himself as such”, is a very bad definition. It is a common consensus to talk about the British empire in the 19th century despite the fact that their rulers never stilled themselves as anything other then king and queen.
A much better definition of empire is “a nation that rules or or excersise significant influence on several other nations”. In that way we can address the content of a state and not merely its form. This gives room for the term “The American Empire” that refers to a republic that have exercised a significant influence in the world in the later half of the 20th and first half of the 21st century. It is in other word not merely a derogatory slur to call America as such, but a simple descriptive fact.
To summarise, any sovereign state that stilled itself an empire, considered itself as the successor state to Rome, this meant that it claimed universal sovereignty. Only two sovereign states have clammed this. The Byzantine empire that existed until 1453 with the fall of Constantinople, and the Holy Roman Empire, that existed until 1806 when it was dissolved by Napoleon Bonaparte the first.
However with the treaty of Westphalia of 1648 that ended the 30 year war, this claim of universal sovereignty was abandoned in favour of our present standard “national sovereignity”. this treaty officially ended the classical imperialism in Europe, where one state claimed the right to rule the whole world.
Colonial imperialism and modern imperialism is different from classical imperialism in that these do not actually seek world domination but rather to expand their host nations influence as much as possible at the expense of all other nations. Modern imperialism differs from colonial imperialism in that it does not seek to establish direct colonial control but rather to coerce its will though more subtle means.
The Islamic imperialism under the caliphate is closely connected to classical imperialism as the title of Caliph means that one are or claim to be the sovereign ruler and supreme pontiff of all Muslims. This idea was abandoned officially in 1924 with the abdication of the last caliph Abdülmecid II.