Many dances often specifiy the type of attire appropriate for that dance. In case you get confused, we are here to help explain the definitions.
White Tie means ultra-formal. Men wear full dress with white tie, vest, shirt and cumberbund. Ladies wear long gowns.
Black Tie means formal. Men wear tuxedos. Ladies wear cocktail, long dresses or dresy evening separates.
Formal usually means the same as Black Tie, but in some cities it could mean a black shirt, no tie with a tuxedo. Ladies wear cocktail, long dresses, or dressy evening separates.
Black Tie Optional means men have the option of wearing a tuxedo or a dark suit. Ladies wear cocktail, long dresses, or dressy evening separates.
Semi-Formal means that men are not required to wear tuxes, but usually dark suits. Ladies wear cocktail dresses, short dress or dressy suit for her.
Cocktail Attire means dark suits for the men and short, elegant dresses for ladies.
Dressy Casual means no jeans or shorts. Men wear sports coats or long sleeve shirts and ladies wear dresses and nice separates.
Casual generally means anything decent goes but when associated with a wedding or special event, good taste should prevail.