dist-upgrade vs upgrade
This command installs the latest versions of any out-of-date packages on your system. It never installs a package that is not yet installed.
apt-get dist-upgradeThis command installs up-to-date version of packages, and may install additional packages.
if you want to do a distro-upgrade, use dist-upgrade. For instance, moving from Debian -stable to Debian -testing. Dist-upgrade is a special upgrade that is used if you are fetching packages from a new location, which is specified in /etc/apt/sources.list
If you want to simply upgrade the packages you have installed for your current distro, use 'apt-get upgrade'.