A wedding announcement is Not necessarily an invitation (or a solicitation for gifts).
If you have a little extra in the wedding budget you can send a wedding announcement out a few months before the wedding to let friends and family know about your engagement and future wedding plans - but NOTE - these are usually sent to people that will be invited.
You can send a post wedding announcement as well, or even an eloped wedding announcement , but these are usually sent to people who weren't invited (for what ever reason).
For tips on how to not make your post wedding announcement seem like a gift solicitation see sample wedding announcements
You will also find help with wedding announcement wording and phrasing .
And obviously a wedding invitation is an invitation, so these are only sent out to those who are invited to the wedding.