what is the difference between update and upgrade? [closed]

Question

In apt-get command, what is the major effects of update and upgrade? I didn't see any difference besides the wording. It seems like update is about system package whereas upgrade is about software package. If that is the case, how to tell which is system package and which is software package?

Answer 1

From man apt

  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...

...

  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

In simple words: apt-get update will update apt's database, the list of available packages (and version's date and number). Where apt-get dist-upgrade will upgrade all packages installed in your system (... if date and/or version differ).