APT and dpkg (Debian, Ubuntu)

In short: dpkg installs individual .deb packages. APT is the layer above: it downloads packages from sources, resolves dependencies and updates the system.

In more detail: apt is the user-friendly command line. Older scripts use apt-get and apt-cache. All commands for installing and removing need root rights (sudo).

In detail

sudo apt update              # refresh package lists
sudo apt upgrade             # update installed packages
sudo apt full-upgrade        # also remove/reinstall packages if needed
sudo apt install vlc         # install
sudo apt remove vlc          # remove (settings stay)
sudo apt purge vlc           # remove including settings
sudo apt autoremove          # no longer needed dependencies
apt search word              # search
apt show vlc                 # details
dpkg -i file.deb             # install a single file
dpkg -l | grep vlc           # list installed packages

• Configuration: /etc/apt/sources.list and /etc/apt/sources.list.d/ (new: .sources files in deb822 format).
• Keys: packages are signed with GPG keys, stored in /etc/apt/keyrings.
• Pinning versions: apt-mark hold package prevents an update.
• Broken installation: sudo apt --fix-broken install.
• Lock: the message “Could not get lock” means another package process is running.

See also: Package management on Linux at a glance, Package sources, repositories and PPAs