Black Tie means Formal. Men wear tuxedos, women wear cocktail or long dresses.
White Tie means ultra-formal. Men wear full outfit, with white tie, vest and shirt. Women wear long gowns.
Black Tie Optional or Black Tie Invited means you have the option of wearing a tuxedo, but it should clue you into the formality of the event, meaning a dark suit and tie would be your other option. Women wear cocktail or long dresses.
Creative Black Tie leaves room for trendy interpretations of formal wear. He can go more modern with a tux — maybe a black shirt, no tie. She wears long or short dresses (maybe a long lace or sequined skirt with a sleek cashmere sweater).