Improvements to both versions of Safari include more accurate Top Hit and Top Sites results, making sure Flash 10.1 content does not overlap other content, more reliable pop-up blocking, improved stability when typing into search and text input fields on Netflix and Facebook, and more stability when using VoiceOver.
Apple's release notes hint that there may be other minor fixes in the update (as usual).
As for the security aspects, all 27 of the addressed vulnerabilities are in WebKit, so the fixes will also avoid exploits via other applications that use the framework.
Please read on for more information about the security fixes.
All the security fixes appear to apply to Safari 4 and 5 on Mac OS 10.4 Tiger, 10.5 Leopard, 10.6 Snow Leopard, and Windows 7, Vista and XP.