There are various placements in which the announcements can show up - in the middle of the screen, on full screen, on the top bar, in the footer, etc. Click here to see every place in which an Announcement can be displayed.
Click here to see the response buttons.
It depends only on the administrator.
In some Announcements, this may be required by him. In part, there will be the "Dismiss" button, which means that if you click at that, the announcement will be displayed to you at a different time, eg. after the next login.