Informal Daytime: Short dress or suit (business attire OK for morning weddings)
Informal Evening: Cocktail dress (which is any dressier dress -- for example, a little black dress -- that's not full-length.)
Semi-Formal Daytime: Short dress or suit
Semi-formal Evening: Cocktail dress
Formal Daytime: Short dress or suit. Hats and gloves optional (although it's rare to see hats and gloves at most weddings.)
Formal Evening or Black-Tie: Long or dressy short cocktail (beading, glam accessories,
Ultra-formal or White Tie: Long gown, extra glitz (furs, diamonds, etc.)