WHITE TIE:
Means ultra-formal.
Men wear full dress, with white tie, vest, shirt and cummerbund.
Women wear long gowns.

BLACK TIE:
Means formal.
Men wear tuxedos.
Women wear cocktail, long dresses or dressy evening separates.

FORMAL:
Men can wear Black Tie, but  could mean a black shirt, no tie with a tux.
Women wear cocktail, long dresses or dressy evening separates.

SEMI-FORMAL:
Men …tuxes are not required,
Women…nor are long dresses.
If the event is after 6pm it becomes more formal!

COCKTAIL ATTIRE:
Men wear dark suits.
Women wears short, elegant dresses.

DRESSY CASUAL:
Similar to business casual, but a tad dressier.

CASUAL:
Generally means anything goes. You just want to look tidy.