The second update is the second Security Update of 2008 and includes security fixes for AFP Client/ Server, Apache, AppKit, Application Firewall, CFNetwork, ClamAV, CoreFoundation, CoreServices, CUPS, curl, Emacs, file, Foundation (one of the fixes here was initially reported by the developer of MarsEdit), Help Viewer, Image Raw, Kerberos, libc, mDNSResponder, notifyd, OpenSSH, pax archive utility, PHP, Podcast Producer, Preview, Printing, System Configuration, UDF, Wiki Server, and X11.
As mentioned above, both of these updates require your Mac to restart in order to perform the installation (downloading can be done without a restart, but given what happened with the last security update that may not be a good idea). In total Apple has released 89.5 MB worth of software updates today (50.5 MB for security, 39.0 MB for Safari). In my case I’ll wait until Friday (when I have no school) to even think about making a backup, downloading the updates, installing the updates, and restarting for the updates to take effect. Also note that almost each Tuesday this year Apple has been releasing new software updates.Enjoy, Alex.