There is a lot of very detailed and specific etiquette surrounding wedding invitations and announcements. The invitation is by far the more common of the two, but the announcement also has a place and time. Woe to the bride who mixes up the two, because you can inadvertently cause some very hurt feelings. Depending on your personal situation, you may find that you need just invitations or just announcements, or some of each.
In a nutshell, the invitation is what you send when you are asking someone to attend your wedding. It sounds simple enough, right? And yet, there are plenty of couples who make the mistake of trying to use an invitation to do the job of an announcement. Wedding invitations should be sent out well before the wedding, to allow the guests ample time to respond, make travel arrangement, shop for a new dress and jewelry for the wedding, buy you a gift, and so on. If you wait until the last minute to send an invitation, there is a strong chance that the recipient will deem it an insincere offer or think that they were on your "B" list (if you remember that backwards episode of "Seinfeld" with the wedding in India, you know just what I am talking about). There is nothing ruder than sending out a very late invitation in the hopes that the guest will be unable to make it, but will send a present anyway!
Some couples unwittingly send this message when they try to use a wedding invitation to do the job of an announcement. The entire purpose of a wedding announcement is to let people who would be interested know that a wedding has taken place. Note the past tense. You do not send out announcements before the wedding under any circumstances. To do so risks people misunderstanding the mailing and thinking that they are invited guests. It's an unfortunate scenario that is best avoided. You don't want to be the one getting the phone call and having to tell someone that, no,they in fact are not invited to your upcoming nuptials!
Although many people are unfamiliar with them, there are many times when an announcement is very useful. Let's say that you had a very small wedding in Tahiti, or eloped to Vegas. A wedding announcement, sent after the fact, is the perfect way to share your good news with your extended network of friends and family. An announcement can also be appropriate when you have had a more traditional wedding, but could not invite every last person that you or your family happen to know. This can be particularly handy if your parents have a large social network or group of business contacts with whom the bride or groom have no personal relationship. You are spared the expense of feeding a roomful of virtual strangers, and they are spared the expense of travel, buying a new dress and wedding jewelry, and the cost of a gift. Yes, I did say gift; there should be no expectation of anything more than a congratulatory note from those to whom you send wedding announcements.
The difference between a wedding invitation and a wedding announcement is actually a very large one. To communicate the intended message, it is important to use the right tool. Once you have pondered your own wedding and your guest list, you will be much better prepared to order the stationery that will help make your wedding a success.