what is the difference between apt-get update and apt-get upgrade

`apt-get update` is used to update the package lists for upgrades and new packages that are available in the repositories. It downloads the package lists from the repositories and "updates" them to get information on the newest versions of packages and their dependencies.

`apt-get upgrade`, on the other hand, is used to actually upgrade the installed packages on the system. It installs newer versions of packages that are already installed on the system. It also installs any required dependencies for these packages.

In summary, `apt-get update` updates the package lists, while `apt-get upgrade` installs newer versions of the already installed packages.