APT (Advanced Package Tool) Advanced Package Tool
Debian/Ubuntu's package manager — apt install nginx. Handles dependencies, repositories, GPG signing on top of low-level dpkg.
Two CLIs: the old apt-get (stable scripting API), the new apt (nicer interactive UX, since 2014). Repositories are configured in /etc/apt/sources.list*. PPAs (Personal Package Archives) on Launchpad let users publish their own packages. Modern alternatives for CLI/userspace tools: Snap (Ubuntu default since 22.04), Flatpak, Homebrew. Competitors: dnf (Fedora/RHEL), pacman (Arch), zypper (openSUSE), apk (Alpine).