The dress code for an evening occasion will usually be specified as white tie, black tie or, less frequently, lounge suits.
White tie is the most glamorous and formal of eveningwear, and is usually reserved for official official functions such as state banquets and ambassadorial functions. Ladies would usually be expected to wear a ball gown.
Black tie can be used to describe formal evening dress generally, but it requires a black dinner suit with a white dress shirt and black bow tie. For ladies, a cocktail dress (long or short) is appropriate.
Lounge suits are worn where formal dress is not necessary, and generally a suit, shirt and tie are still expected.