Can you please categorize the different attire formalities and what each one of those mean for guests?

Wedding Planner Expert, Jessica Adler of JA Special Events and Weddings answers this:
Knowing what to wear to a wedding or special event can be pretty confusing nowadays. Based on the time of year, time of day, and part of the country, as well as the style of the venue, appropriate attire can really vary. It can really be helpful to look online and learn about the town and location for the wedding in order to really select your outfit. In general, I recommend guests be dressed similarly to the bridal party and immediate family or one step down if it is a more formal, traditional wedding. For example, if the wedding party is in full-length gowns and tuxedos, a guest could wear a nice, dark suit and a shorter cocktail dress.

Black tie - This is the most formal affair. Traditionally, men would wear a tuxedo, although sometimes a dark suit and tie may suffice. Ladies can wear a floor-length gown or cocktail dress.

Semi-formal - A small step down from the full-length gown and tuxedo. A full-length dress is appropriate for women though a shorter length is acceptable. Men should be wearing a suit and tie.

Dressy casual or Informal - The emphasis is truly on the "dressy" and not the "casual". Jeans and shorts are not really options, more slacks and a buttoned or polo shirt for men and a pant suit or cocktail or dressier sundress for ladies.

Casual - Unless the event is truly at a ranch or farm and more likely the rehearsal or welcome dinner, even casual really doesn't mean jeans and shorts. This is similar to what you might consider "business casual" at your office. With the right accessories for a less formal event, women can wear a great pair of darker jeans or slacks or a more casual skirt and top. You still want to look dressed up and ready to go out, but not too formal. Men can wear slacks and a nice shirt (should be collared), not a T-shirt.

Fun or themed attire - Some couples throw theme weddings where they want their guests to come in costume. In this case, a themed outfit or certain colors is appropriate. You definitely will want to make sure that what you plan to wear is what the bride and groom have envisioned.

Daytime affair - Again, your attire here would be similar to business casual, but focus on wearing lighter and brighter colors, particularly in the summer season. A sportscoat and no tie for men or just a tie and without a jacket are appropriate. Women can wear a more fun sundress or skirt and top or nice blouse and slacks. The darker, more formal suits and dresses are less common.

Wearing white - Simply put, this wedding planner does not suggest that any women wear a white or ivory dress or pantsuit or really even a white or ivory top or skirt to a wedding. A pattern or print that includes white or ivory is perfectly acceptable.

Wearing the wedding colors - While you don't want to look just like a bridesmaid or like the mother of the bride or groom, it can be fun and flattering for the bride who likely spent much time and effort selecting her color palette to have guests arrive wearing some of the wedding colors whether that be with accessories or your actual dress or outfit.