A queen is a female monarch. There are two major types of queens: a queen consort or a queen regnant.
A queen consort exists mainly as wife of the king and mother to his royal heirs. She will often have little or no power apart from what the king allows her to possess. Most queens throughout history have been queen consorts.
A queen regnant possesses power in her own right. Examples include Queen Elizabeth I and Queen Victoria, to name two.