What to Wear

This was cribbed from an email that said "This is a guideline and explanation of Dress Codes, taken from a recent USA Dance, Inc. Publication, which you may see posted on invitations and announcements to various Public and Private Special Events." 

• White Tie - The most formal category of dressing. For the Gentlemen it means a black tailcoat with matching trousers trimmed by a ribbon of braid or satin on the outside of each trouser leg, a white pique tie, white pique single or double-breasted vest, and a wing-collar shirt with a stiff pique front. White gloves are nice optional accessories for Gentlemen. The Ladies appear in a ball gown, which is an evening dress with a full skirt, possibly with open back and low neck line. Elbow-length gloves are also a nice addition for the Lady.
• Black Tie - Gentlemen in black tuxedo coat, trousers trimmed with satin ribbon along the outside of the leg, cummerbund and bow tie. The phrase "black tie" does not refer to the color of the tie. In fact colorful ties with matching cummerbunds are very popular. Ladies appear in ball gowns.
• Black Tie Optional - Same as above, except Gentlemen have the option of wearing a regular suit with a tie (bow tie preferred), and Ladies wear a cocktail gown or dinner dress. Long to full-length skirts are preferred; short skirts are not recommended.
• Formal - Gentlemen in suit and tie (nowadays a sport coat is often acceptable), Ladies in a cocktail dress or evening dress.
• Semi-formal - Gentlemen in dress slacks with a dress shirt and tie, jacket is optional. Other options include a vest or sweater that shows the tie. At the lower end of formality these events can be attended without a tie, e.g. with a turtleneck and jacket. Ladies in evening dress or dinner dress, but other chic outfits are also acceptable, like flowing pants, etc.
• 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.
• Casual - generally means any thing goes, jeans and t-shirts included.

 I have not been able to find the source publication mentioned above, but here is a site with a very expanded and detailed dress code guide - lots and lots of info.