The 10.5.4 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

You can read detailed information about the update at the following Apple Support document.
For detailed information about the security updates, please read this document.

T0.5.4 comes just over a month after the May 28 release of 10.5.3.  This latest update contains several key additions to the Mac operating system to  take advantage of Apple's upcoming MobileMe service (formerly .Mac)

Safari 3.1.2 for Tiger
Apple also released Safari 3.1.2 for Mac OS X Tiger 10.4.11. This update addresses a WebKit vulnerability that was originally fixed in Safari 3.1.2 Windows XP and Vista (released June 19th).