Usually, "queen" is a term that refers to a woman ruler in Europe, or sometimes in Africa. "Empress" is usually a term that is used to refer to a woman ruler in Asia or the middle east. Both "Empress" and "Queen" are used in monarchies. To an American's ears, if you said "queen" they would like about the queen of England. If you said "Empress", they might think about the empress of Japan. This is not always the case, but this is usually the difference.

The difference between a Queen and Empress is that, the Queen is the wife of the monarch or could be a Queen Regnant of a country. While The Empress is the Wife of the reigning Emperor