A Black Tie is a call for formal attire. Men usually opt for tuxedos, women wear cocktail long dresses. A little black dress is completely appropriate for black tie functions getting you all the attention. Formal refers to black tie usually but in some cities it could mean a black shirt or no tie with a tux.
An invitation from White Tie or Ultra-formal requires men to wear full dress, with white tie , vest, shirt. Women usually go for long gowns.
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
Creative Black Tie is for trendy interpretations of formal wear. Men can go modern with a tux maybe a black shirt, no tie. She wears long or short dresses, separate dresses for evening events. Themed parties have dress codes with wide variety of creative black tie. In that situation you can have more fun in choosing the dress and trying more mix match.
Resort formal is for warm weather like a beach wedding where the women can wear something bright and lightweight for example: floral sundress
Semi-Formal means in which tuxes are not needed, nor long dresses. An evening wedding would still suggest dark suits for him, and a cocktail dress for her. During day semi-formal events mean light colored suit for him and an appropriate short dress suit for her.
Business Formal is quite similar as Semi-Formal for him, but women should opt for more tailored dressy suits and dresses. The idea is to be dressed appropriately which means nothing too sexy or goody but still dressed up.
Cocktail attire means short elegant dresses for her and dark suits or coats for him. The little black dress is the ultimate cocktail dress and appropriate for most special occasions.
Informal is often misinterpreted as same as Casual. But it is similar to Semi-Formal — for him it may be dark suits and short dresses, but for her like opting for little black dress especially when associated with a wedding or special event .
Festive Attire usually comes around the holidays. Being in high spirits of partying one can be dressed up in Informal or Semi-Formal. For her, it means to choose looks with a bit of sparkle .
Dressy Casual means dressing up for casual look. For him, it could be trousers and a coat, for her dressy pants look.
Jeans, shorts, T-shirts and other casual looks are not appropriate for Dressy Casual.
Casual generally means dressing up anything like jeans, baggy pants, etc. If the host or hostess hosting an event wants a more dressed-up look then it would be decided by the invite. If in doubt you can always ask the host or hostess what they expect their guest to be dressed like.