WHITE TIE
This is the most formal of all wedding dress codes (think state dinners and the Oscars).
He should wear: A long black jacket with tails, a white pique vest and a bow tie. Formal black shoes and even white gloves are appropriate.
She should wear: A formal, full-length ball gown. Glamorous makeup and dramatic jewelry and hair are appropriate.

BLACK TIE
This is the next most formal wedding dress code and usually means the wedding is an evening affair.
He should wear: A tuxedo. A black bow tie, cummerbund and patent leather shoes are also suggested.
She should wear: A chic cocktail dress or a long evening gown. The couple, wedding party or close friends can help answer questions about the appropriate dress length.

BLACK TIE OPTIONAL / FORMAL
The wording here suggests something slightly less formal than black tie. This means that a tuxedo isn't required, but the event is still formal enough for one to be appropriate.
He should wear: A tuxedo or formal dark suit and tie.
She should wear: A long evening dress, dressy suit or formal cocktail-length dress.

BEACH FORMAL
This suggests an elegant beach wedding—so dress to impress, but also dress for the elements (sun, sand and water). Anything you'd wear to a nice restaurant on a summer night is appropriate.
He should wear: A summer suit with a linen shirt (no ties required), linen pants (or khakis) and sandals.
She should wear: A formal summer sundress at tea or knee length with flat sandals. Makeup and hair can be natural.

COCKTAIL ATTIRE
This is slightly less formal than black tie and black tie optional, which means no tux required for men and no floor-length gown required for women (unless you'd like to wear a long dress, which is popular for winter wedding attire). 
He should wear: A suit and tie. Lean toward darker hues in chillier months, and feel free to opt for lighter grays or blues in warmer weather.
She should wear: A cocktail dress or dressy suit or jumpsuit.

SEMI-FORMAL / DRESSY CASUAL
Depending on the time of the event, you'll want to dress somewhere between formal and casual. Wear darker, more formal hues for an evening fete; opt for light colors and fabrics for a daytime wedding.
He should wear: A suit and tie, dark or light depending on the season and time of day.
She should wear: A cocktail dress or a dressy skirt and top. 

CASUAL / DAYTIME
Generally, casual means anything goes—but jeans, shorts and tank tops are probably not appropriate unless they're specifically noted as acceptable. For the purposes of wedding wear, assume business casual to be on the safe side.
He should wear: Dress pants with a button-down shirt or polo.
She should wear: A summer sundress or a skirt or pants with a nice blouse. Makeup and hair can be natural.