apt-get(8) update && apt-get(8) upgrade
APT package handling utility - - command-line interface
    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.
AND and OR lists are sequences of one of more pipelines separated by the &&  and  ||  control  operators,
respectively.  AND and OR lists are executed with left associativity.  An AND list has the form

       command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

       command1 || command2

command2  is  executed  if and only if command1 returns a non-zero exit status.  The return status of AND
and OR lists is the exit status of the last command executed in the list.
    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.
source manpages: apt-getapt-get