Informal Daytime: Dress shirt and pants, preferably a sports jacket.
Informal Evening: Suit
Semi-Formal Daytime: Suit
Semi-Formal Evening: Dark suit
Formal Daytime: Dark suit and tie
Formal Evening: Tuxedo (if invitation states "black tie") or dark suits if women wear short dresses.
Ultra-formal Evening or White Tie: White tie, vest and shirt.
Don't try to get cute with a tuxedo. A black tux with white shirt and black bow tie is the best way to go. If "Creative Black Tie" or "Texas Formal" or some sort of other vague formal description is used, then going with a tux and black shirt, no tie, might be acceptable. Also, trendier cities like Los Angeles and New York might be more accepting of breaking with tradition.
Do wear a dark suit, with a tie if the wedding is after 6 PM, and doesn't say "Black Tie."
Don't wear a tuxedo during the day time, regardless of the formality of the event.
Do use good judgment if the invitation doesn't specify the formality of the event. A dark suit and conservative tie will take you just about anywhere.