What they are – Announcements sent after the wedding to anyone who wasn’t invited to your wedding.
When you send them – Some traditions dictate the bride and groom should mail these on the way to the reception from the wedding. The next day is also fine.
Do you need them? – You only really need these if you had a destination wedding or were unable to invite many people who would normally have made your guest list for another reason such as budget. You may also announce your wedding in the local newspaper.