Rootkits are, basically, hacking tools, that are designed to allow others access to computers and to conceal their existance and activity. The fun thing about this rootkit was that it could not be uninstalled, and that it broke your system if you did try to remove it.
After this story broke, Sony has released a piece of software to remove their rootkit, but still -- apparently it is OK for content companies to break your system in non-trivial ways, just so you can't play their CDs on your computer. It's even more fun that it is illegal under the DMCA to tell others about this and how to disable/uninstall the rootkit.
DRM is evil, and Sony is an evil company.