Update: Update is used to resynchronize the package index files from their sources. The indexes of available packages are fetched from the
location(s) specified in /etc/apt/sources.
Upgrade: Upgrade is used to install the newest versions of all packages
currently installed on the system from the sources enumerated in
/etc/apt/sources.list. Packages currently installed with new versions available are retrieved and upgraded; under no circumstances are currently installed packages removed, or packages not already installed retrieved and installed. New versions of currently installed packages that cannot be upgraded without changing the install status of another package will be left at their current version. An update must be performed first so that apt-get knows that new versions of packages are available.
Dist-upgrade: Dist-upgrade in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a “smart” conflict resolution system, and
it will attempt to upgrade the most important packages at the expense of less important ones if necessary. So, the dist-upgrade command may remove some packages. The /etc/apt/sources.list file contains a list of locations from which to retrieve desired package files. See also apt_preferences(5) for a mechanism for overriding the general settings for individual packages. And with the newer apt tool available from 14.04 onward.
Full-upgrade: Full-upgrade performs the function of an upgrade but may also remove installed packages if that is required in order to resolve a
#apt-get update && apt-get dist-upgrade