This question gets asked here pretty often. Not to discourage further discussion, but the basic idea is that kings (queens) rule over homogenous states, while emperors (empresses) rule over multiple states/nationalities/races (variously defined over time). So Queen Victoria was, for example, Queen of Great Britain and Empress of India.
Caveat: The monarchical titles (prince, king, emperor) have been applied wildly differently over time and place and are often changed/lost in translation.
Here are some past threads:
https://www.reddit.com/r/AskHistorians/comments/2ygnra/what_is_the_difference_between_a_king_and_an/