If you have been using debian/ubuntu flavors for a while you have come across updating your system that has kept back a random amount of files. This happens because there is a image update available. By doing this it is only updating the image. You wont be upgrading to a different version of the server eg: 12.04 to 12.10. This will keep me at 12.04.
sudo apt-get dist-upgrade
Once the upgrade is complete you may have to reboot the server. It's always a good idea after you upgrade you update apt-get so run
sudo apt-get update
I also after doing the upgrade run the autoremove command. This will remove any packages that are no longer required and used. a good way to keep your server clean with out clutter.
sudo apt-get autoremove