The 10.8.4 update is recommended for all OS X Mountain Lion users and has features and fixes that improve the stability, compatibility, and security of your Mac, including the following:

Compatibility improvements when connecting to certain enterprise Wi-Fi networks

Microsoft Exchange compatibility improvements in Calendar

A fix for an issue that prevented FaceTime calls to non-U.S. phone numbers

A fix for an issue that may prevent scheduled sleep after using Boot Camp

Improved VoiceOver compatibility with text in PDF documents

Includes Safari 6.0.5

For detailed information about the security content of this update, please visit: http://support.apple.com/kb/HT1222

See this article on how to validate the authenticity of this update.