There is a clear line of difference between an emperor and a king. While a king heads a kingdom, an emperor heads territorial space bigger than a kingdom. An emperor necessarily doesn't need to be a king, e.g., Bonaparte Napoleon of France was emperor of France with no royal bloodline. However, a king can rise to become an emperor by conquering other territories around and beyond to expand his kingdom to the stage of becoming an empire, e.g., Alexandre the Great of Macedonia expand the Hellenistic Kingdom by conquering the whole of Greece before expanding his kingdom down Asia, thus becoming both a king and emperor.