Sharepoint:SPSecurityTrimmedControl
is a wrapper control you can use to limit who can see certain sections of a Sharepoint page. My interest is in hiding the Ribbon, The blue bar at the top of most Sharepoint 2010 site pages.
The first article I found was Hiding the Ribbon for Anonymous Users in a SharePoint 2010 Publishing Site by by matdesmarais
A more in depth article is How to: Hide the Actions Menu in SharePoint. Notes at the bottom of this article refer to Microsoft reference sites that discuss parameters and atributes.
A related article is Real World Branding with SharePoint 2010 Publishing Sites written by Andrew Connell and Randy Drisgill