This convenient control has been around since the old good MOSS days, but was never given enough attention..
So i would like to explain what it is good for and how you can use it in your site.
SPSecurityTrimmedControl will basically hide all its content from users that won't meet a given security definition.
Unlike the previous solutions (style.display='none'...), the content will be hidden on the server side!
For Example: recently i was asked to show the top ribbon only to users with Manage rights.
What i did, was:
Open the site's Master Page in SharePoint Designer
Find a div with id= s4-ribbonrow, its content is actually the top ribbon
Wrap this div with a SPSecurityTrimmedControl, like this:
Useful attributes of the control:
(possible values: CurrentSite, CurrentList, CurrentFolder, CurrentItem or RootSite)
(possible values: All or Any)
(possible values are from the enumeration SPBasePermissions:
(possible values: AllUsers, AuthenticatedUsersOnly, AnonymousUsersOnly)
save, check-in, publish and there you go!