Traditionally, the wedding announcement was sent to all friends, family and acquaintances, all of whom were not necessarily invited to the actual wedding ceremony. In the past, the parents of the bride would send out announcements the day after their daughter was married in order to let all of their friends know that she had changed her name, etc.
Formerly, the wedding announcement was sent out after the wedding, where the wedding invitation was sent out prior to the wedding only to individuals who were invited to the ceremony.

However, a more cost-effective and inclusive use of the wedding announcement has developed in recent years. Now, a bride and groom can prepare a list of several hundred (and even into the thousands) friends, family and casual acquaintances. The wedding announcement goes out to the entire list, usually inviting them to an open-house style of reception.
Here’s the cost saving part…the printer then makes “insert cards” that are intended only for certain guests on the mailing list. An insert card may be used to invite specific guests to the ceremony or the wedding breakfast. When all of the announcements are stuffed in the envelopes, you can go through and only place insert cards into the ones that you want to have at the ceremony or other exclusive event.

You can also use insert cards to notify your guests of where they can find your bridal registry to make it easier for them to shop for your wedding gift (or send a wedding gift if they are unable to attend). Using the insert card for the gift registry also keeps that information off of the announcement/invitation itself so that it doesn’t distract from the other information.
A wedding invitation was what you send to invite potential guests to your wedding. A printed wedding announcement was intended to only announce the event has happened. It used to be that there was no obligation to send a wedding gift after having received a wedding announcement. However, with this new combination announcement and invitation, there is a higher expectation to send a gift when only the announcement is received.

Most modern printing companies use the terms announcement and invitation interchangeably as a direct result of more and more brides combining the two in order to save money. Another item that is commonly confused with the wedding announcement, is the “save-the-date”. Save-The-Date cards are more common with long engagements (6 months or more). Traditionally, people would send out a save-the-date card well in advance of the wedding to those guests that they wanted to have at the wedding. Then as the wedding draws closer, they would send out a formal invitation to only those guests invited to the ceremony and dinner. Then the day after the wedding, they would send out a wedding announcement to notify all friends, family and acquaintances.

If you do not have a long engagement, you now only have to do one mailing that encompasses all three of these mailings. Sent out early enough, it serves as a save-the-date, it serves as the announcement, and with the insert cards, it also serves as the invitation.