When in doubt, you can always ask the happy couple (just try to do it at least a week before the big day) about their vision for dress. If they come back with "White Tie", "Black Tie" or "Semiformal" and you're not sure what that means, you can refer to the chart below! You could also ask what the wedding party will be wearing and try to match their level of dress accordingly. This does not mean you should wear the same exact dress or suit as the bridesmaids/groomsmen, but you can match the formality level and even pick something in the color scheme. You don't want to out-dress them, but you also don't want them all to feel too over-dressed (or awkward, in the case that you're a bridesmaid lookalike, and aren't a bridesmaid) around you. On that note, you also don't want to wear anything too sexy, racy or tight. Many of your college pals may be there, but don't be confused- this isn't a night out on the town where you cover up what you must and show what you can. Look nice, but class it up a little! How often do we get to dress up like this?! If the outfit you wanted to wear is too big, too small, torn, has holes or stains, leave it at home.
Jeans: Unless you hear otherwise directly from the bride and groom (and it's unlikely that you will), blue jeans and cargo pants are not appropriate for a wedding. I've seen a number of individuals in jeans at weddings throughout the past wedding season, and they just looked out of place and...underdressed. A wedding is a very special occasion, and while you may be most comfortable wearing jeans, it may make others feel uncomfortable- or worse, disrespected- if you wear them. Please do yourself a favor and make a better choice. It's always better to err on the side of being too dressy than not dressy enough. Please also note that in most cases, sneakers/tennis shoes are not acceptable. Plan ahead and go get yourself some nice shoes for their big day! If you choose well, you'll be able to wear them again for another event in the future. Go for comfort and diversity- something that will go with many different outfits and colors- if possible.
Color choice: You should not wear solid white or ivory to a wedding. There should be no confusion between you and the bride. It's also been said that you should not wear black, because in the past, that showed that you did not approve of the marriage. I think that this is just a thing of the past now, as I've seen many people wear black in an appropriate and classy way, and if they didn't approve of the wedding, I'd hope that they wouldn't be there. I did read that it's still poor etiquette to wear black to a daytime wedding, so you may want to keep that in mind and make your choice accordingly.
Weather: You should, of course, consider the weather when planning your outfit. If you'll be in a cold location, you'll want to plan a top layer, such as a jacket or shawl. You'll also want to consider stockings or leggings of some kind, and may even want a pair of closed-toed shoes. If it's a warmer location or the wedding is taking place in the hotter Summer months, a breathable outfit with a layer that you can take off is nice. There's nothing more uncomfortable to watch than a Summer wedding where the bridesmaids have sweat dripping down their legs and the guys look like they need to take off not only their jackets, but their button-down shirts, too.
Location: Outdoor weddings may be held in a grassy area, so high-heeled shoes may not be the most logical plan. Indoor weddings at a country club or fancy venue call for a more tailored, fitted, fancy-looking outfit (Read: Your maxi dress is really pretty and fun, but might be better for a Pavilion wedding at a park than a country club reception). If the ceremony is being held in a church or synagogue, be sure to cover up bare shoulders, with either a shawl or a jacket.
Socks: Guys, you may not think it's noticeable when you put on that pair of white ankle socks with your fancy dress shoes, but it is. Plan ahead and wear a pair of black socks, or go for a colorful pair with a cool pattern- extra points if they match the wedding's color scheme! Just leave your everyday and workout socks at home. K thanks. It should go without saying, but undershirts should be white, clean, tucked in, and new-looking. Boxers and underwear should not be visible when you walk, bend or dance, and should also be clean. You never know who will see them.
Cost: When you send in that RSVP, you're agreeing to put your best foot forward and be there- in every way- for your friends, coworkers or family members for their big day. This means that if it's a black tie affair and you do not have a fancy floor-length gown or tuxedo/nice suit, you should start looking and budgeting for one as soon as possible. Weddings can be costly for all involved, but the expense will be worthwhile when you see the smiles on those 2 faces and you get to be part of the celebration.
For additional levels of formality, see Emily Post's Attire Guide. Note: I did notice that the Casual section of this post says that Good Jeans and Cargo Shorts are acceptable. Use your best judgment, and while I still don't feel it's appropriate, if you should make that choice, just be absolutely sure that the dress code is Casual. Never assume if you're not sure.
Still not sure what to wear? Check out http://www.dressforthewedding.com/ for more ideas and even to find the dress or suit you're looking for! For great guidelines and suggestions, this page is fantastic!