When a ruler rules a country, it is known as his kingdom. But when an emperor rules, he will be ruling a group of countries, and these are collectively known as the Empire. The Roman Empire is such an example. Another example to differentiate between the empire and kingdoms is the rule of the British Empire in India. The people from the British Empire were ruling the kingdoms in India while there were many small kings in the country.
Therefore:
1.Emperor is higher in rank and honor than the King.
2.Ruler rules a country, while emperor rules a group of countries.
3.An emperor rules an empire, while a ruler rules a kingdom.
