There are a number of software notices to display message(s) to broad sets of Wikipedians.
Shows to all anonymous users; if blank, the sitenotice shows to anons as well. Appears above the page title. Controlled by MediaWiki:Anonnotice. Not dismissable.
Shows to all users who are able to edit the page. Appears above the edit textarea. More information available at Wikipedia:Editnotices.