Like the other updates to Debian stable, the 6.0.7 Squeeze update is mostly to fix security problems along with taking care of "serious problems" in the distribution.
Among the changes for Debian 6.0.7 is a new version of the ClamAV software, a security fix for the kFreeBSD-8 kernel, and updating the Linux kernel to Linux 18.104.22.168 with a few back-ported updates.
There's 78 security issues within packages that were taken care of for Debian 6.0.7 by Debian's security team.
More release details on Debian 6.0.7 can be found via the Debian.org news announcement.
If you are curious about what's coming into the next major version of the Debian operating system later this year, see the Debian 7.0 "Wheezy" features that were written about a few days ago.