Wedding announcements differ from wedding invitations in that they are sent after the wedding has occurred. They can be sent for many different reasons. For example, the couple may have had a destination wedding with only a small number of guests, but want to announce their union to those who weren't invited. Or if the couple eloped on the spur of the moment, they would want to get the word out about their newly married status.
Wedding announcements are often less formal than invitations. Details can be printed on them if desired, or they can simply be written out on a blank card if you are only sending a small number of them. They could also include a card with the couple's names, address and phone number so that friends and family can make a note of the new information. Generally, the date and location of the wedding is inluded in the wedding announcement, but not the time or address.
You can also go the formal route with your announcements if you want. Adding language such as "Mr. and Mrs. John Jones and Mr. and Mrs. Joe Smith announce the marriage of their son and daughter…" gives the announcements a formal air. You could even have a wedding invitation reworded to turn it into an announcement if you find one you are particularly fond of.
Wedding pictures make a nice addition to wedding announcements. They can even be printed on the announcements themselves if you like. These add a personal touch and provide those who weren't there with a glimpse of your wedding.
Wedding announcements provide a way for couples to inform those who were not in attendance at the ceremony of their newlywed status. Whether formal or informal, they provide friends and family with a beautiful reminder of the couple's union.