In Pictures: OS X Mountain Lion arrives - the top ...
Web-only guest access
Apple has long provided a guest account in OS X, so you can make sure the kids or babysitter aren't messing in your files, applications, and websites. OS X Mountain Lion provides a second guest option: You can have OS X Mountain Lion boot into a Safari-only mode, in which only Safari runs -- an isolated copy that stores nothing when it is quit.
You can't have both guest options active, though you can always create a limited standard account for trusted guests and use the Safari-only mode for everyone else.
In Pictures: OS X Mountain Lion arrives - the top 25 features