If an invitation to a formal event specifies white tie (such as a ball or very swanky evening reception) men are expected to wear:

• full dress tails (i.e. a tail coat that is cut short in the front and long in the back)
• a white waistcoat (vest, for americans)
• a white shirt with a wing collar
• a white bow tie
• black dress shoes

Tail coats are normally black, and should be worn with black trousers.

Women are expected to wear a long dress, which may be off the shoulder. Elbow length gloves are appropriate. Break out the diamonds.

White tie should not be worn in the day.