This latest shield for your Mac includes dozens of fixes and patches, such as a series of new defenses against a wide-variety of malicious PDF files, H.264 movies, and disk images, all of which could potentially execute arbitrary code. There is also a repair for DirectoryService, which has a memory corruption issue that can allow entry for a remote attacker. OS X's Certificate Assistant gets a boost, preventing users from falling prey to domain attacks. Servers will benefit from a beefing up of the Adaptive Firewall, which previously might not have detected brute force attacks and login attempts using invalid user names.
As usual, you can download the update via OS X's Software Update mechanism. Mac users often pat themselves on the back for not having to deal with the viruses, trojans, and worms that plague their Windows friends. But it pays to be vigilant. Given the range of important updates in this round, even the lackadaisical among you should install the latest fortification from Apple.