The difference between an emperor and a king is that an emperor rules over an empire and a king rules over a kingdom. But these are just titles: there is no functional difference between an empire and a kingdom.
In the Roman Empire the emperor had both a high priest role and a kingly role. Thus, many throughout history have distinguished emperors from kings on the ground that emperors have a religious function whereas kings do not. King Henry VIII of England said he was an emperor in the sixteenth century because he rejected the authority of the Pope and called himself Head of the Church of England. But Henry VIII and his successors never officially changed their title to "emperor." Nor did other emperors always have a religious role. The Holy Roman Emperor, for example, had no religious authority.
When Queen Victoria became "Empress of India," she took a manufactured title. She was not an empress because she was queen of many kingdoms. She was only Empress of India. In fact, the primary reason that India became an "empire" was because Victoria's daughter married the German Emperor, and Victoria did not want her daughter to have a title which she lacked.
随便搜了下，比较详细的介绍：
e和k只是头衔不同，本质上没有大的功能区别
e可以扮演宗教和行政2角色，而K一般只担任王国领袖
维多利亚Q因为女儿嫁给德E所以自己也封了个E。。。因为不想比她女儿少个头衔
