That's a great question....I'm so glad you asked! This is definitely a question that gets asked over and over again. And along with this, we'll touch on the subject of WHEN you wear a tux and WHEN you wear a suit.
The main difference between a tux and a suit is the presence of satin. On a suit, the coat, pants and vest all are made of the same material...wool, polyester, linen. The buttons on a suit generally are a plastic or a covered button in the same fabric as the suit. A tux traditionally has some satin facing on the lapels, the buttons and trim on the pockets, and sometimes there is a stripe or bead of satin down the leg of the pants. It used to be where a bow tie and vest or cummerbund went only with a tux and a long tie went with a suit. But the times, they are a-changing.... Now bow ties and long ties with vests work with either tux or suit. Cummerbunds with bow ties, however, go with tuxedos. With a tuxedo, a pleated shirt with either a wing or lay down collar is traditional, but now a nice microfiber shirt with a laydown collar is also an option and acceptable. Of course, black patent leather shoes go with a tux and flat black shoes go best with a suit.
Now, the next question is: When is a tux appropriate....and when is a suit? Rule of thumb is if the woman is wearing a gown, then you should wear a tux. If the woman is wearing a dress, then a more casual suit fills the bill. The tuxedo is formal. The suit is more casual. You wear a suit to work, church or as a guest of the wedding. The tuxedo is formal and sets you apart from the rest of your guests for your special day. Your bride is looking her best, and you should, too, on the day you start your lives together.