An announcement is a public statement that's usually formal and has a specific purpose.
There are many kinds of announcements, but they're all public and meant to inform. In school, the announcements might say whose birthday it is and what clubs are meeting after school. In a store, an announcement might mention a sale or a lost kid who's looking for her parents. The President makes lots of formal announcements, like the decision to run for re-election. So basically, if it's announcing something, it's an announcement.