A black-tie dress code traditionally refers to tuxedos for men and floor-length dresses for women. That being said, as time has gone on, most couples have loosened up their definition of a black-tie wedding just a bit. Black-tie optional weddings usually imply that, while black-tie attire is preferred, men can opt instead for a nice dinner jacket, dress pants, and dress shoes, and women can instead wear a very nice cocktail dress (think high quality fabrics and rich colors). If you go with the later, make sure that your formal wedding attire will blend in with a crowd of people wearing tuxes and evening gowns.
Men:
Ensemble: Tuxedo, dinner jacket, or formal dark suit
Shoes: Dress shoes, dark dress socks
Accessories: Black tie, black bowtie
Women:
Ensemble: Floor-length gown, nice cocktail dress, formal pantsuit
Shoes: Heels in a neutral or complimentary color
Accessories: Classic jewelry (pearls, diamonds, faux diamonds), evening purse or clutch