Wedding announcements are completely optional. Often people choose to send wedding announcements when the celebration is too small or to acquaintances or business associates who, while not particularly close to the family, might still wish to hear the news of the marriage.
Usually it is a handwritten or printed card and much like an invitation, it includes the names of the bride and groom, the date of their marriage and sometimes the names of their parents. The couple should not expect gifts when they send announcements; it is not customary or necessary for the recipient of an announcement to send one.
Just like wedding invitations, the envelopes for wedding announcements should be handwritten. You may handwrite them yourself, or hire a calligrapher to do it for you. It is acceptable to have the envelopes printed if the printing looks like handwriting, but it is not considered proper to use address labels.
Ideally, wedding announcements are sent right after the wedding, but may be mailed up to several months later, although not prior to it. If you are going to be away on your honeymoon, consider preparing announcements prior to the wedding and leaving them with a family member or friend to be mailed right after the ceremony.