There are 3 things in the wedding world or sent items they are , save-the-date, announcements and invites.  Save-the-date is normally sent anywhere from 3-9 months before the wedding to people that will be invited so they know to plan on taking time off, plane tickets and anything else they might need to do to prepare.  Invites are normally sent 2-9 weeks before the wedding to actually invite the person and give the details of where, when and of course where you're registered.  Then there are announcements these are typically sent after the wedding to let all those doubters know you really did tie the knot.  Really they are meant to as you sharing the joy, but really most people send them to brag.  Anyways, some people do send announcements before hand but really they are more of save-the-date.