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.list. For example, when using a Debian archive, this command retrieves and
       scans the Packages.gz files, so that information about new and updated packages is available. An update should always be
       performed before an upgrade or dist-upgrade. Please be aware that the overall progress meter will be incorrect as the size of
       the package files cannot be known in advance.

   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. The dist-upgrade command may therefore 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.