White Tie: Is not frequently used today, but you may (if you are royalty or travel in that circle) see it on an invitation from time to time. This is the most formal of all options. Men it means tails and you guessed it a white tie. Ladies long gowns only and you may want to break out gloves.
Black Tie, Black Tie Required: The most common of formal attire. Men it means a tuxedo and for the women it really means formal. Ladies will wear long gowns, but a formal cocktail length dress is acceptable. Brides keep in mind if you are requiring that people wear black tie your groomsmen must be in tuxedos. Maids should be in long dresses, but I've seen the rule broken.
Black Tie Optional, Black Tie Requested, Black Tie Invited: This is also very common for formal events. Men it means tuxedo if you have one or can rent one, but if not, a black suit is acceptable. Unless you are in the immediate family and will be in formal pictures in which case you should wear a tuxedo. Ladies it's the same as above, but more women may tend to wear formal cocktail length dresses. This is my suggestion if you are having a formal wedding. It may be a really big hassle to have guests go out and rent a tuxedo. Brides if you are using this dress code it's reasonable to assume your groomsmen should be in tuxedos, however, you can be a little more flexible with the tuxedo (i.e. leaving out elements like a cummerbund or vest). Maids can wear cocktail length dresses, but I'd still do long if possible.
Note: Formal and Black Tie Optional mean the same thing.
Semi-Formal: This is the most common of all wedding dress codes. Men this means suit (jacket and tie). Ladies cocktail dresses, leave the sundress and formal gown at home. Brides your groomsmen can wear suits.
Informal: This is typical for an afternoon wedding. Men it means trousers (not jeans) and a collared shirt. Most etiquette books would suggest a suit (dark or light) with or without a tie.
Destination Weddings: In general I'd make sure to consult the bride or host of the event. For the most part men can wear linen pants and a collared shirt and ladies can wear sun dresses. This is going to vary depending on the event and the place.
In general if you are confused about the attire consult the host. If you are going to be included in formal pictures be sure to ask the bride and even then always be on the formal side.
Did I leave something out? What was your dress code?