Formal usually means the same as Black Tie, but in some trendier cities like New York or Los
Black Tie Optional
A Black Tie Optional or Black Tie Invited gives you the option of wearing a tuxedo or formal dress, 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, long dresses or dressy evening separates.
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 or evening separates.
Sometimes, themed parties call for dress codes like Texas Black Tie, or other variations of Creative Black Tie. In that situation, you can have more fun with it, choosing a dressy look with a theme (for him, it could be a tux with boots and for her it could be a long dress paired with Southwestern style silver belt and jewelry).
Resort formal is a designation for warm weather locales (like a beach wedding) where the women can wear something bright and lightweight (floral sundress for example.)
Semi-Formal or After Five means that tuxes are not required, nor are long dresses. An evening wedding (after 6 PM) would still dictate dark suits for him, and a cocktail dress for her. Daytime semi-formal events mean a suit for him and an appropriate short dress or dressy suit for her.
Cocktail Attire means short, elegant dresses for her and dark suits for him. The little black dress is the ultimate cocktail dress and appropriate for most special occasions.
Informal is often interpreted as the same as Casual but it actually calls for the same dress as Semi-Formal -- dark suits for him, short dresses for her -- especially when associated with a wedding or special event.
Festive Attire is usually seen around the holidays, with the mood of the party being Informal or Semi-Formal. For her, it means to choose looks with a bit of sparkle or holiday bent (i.e. a beaded sweater with black pants, a red silk blouse with a black skirt).
Dressy Casual calls for dressed-up versions of casual looks. For him, it could be trousers and a sportcoat, for her a dressy pants look. Jeans, shorts, T-shirts and other casual looks are not appropriate for Dressy Casual.
And when in doubt about what to wear, remember you can always ask the host or hostess what they expect party guests to wear.
Courtesy of About.com.