The package manager used by Debian and Ubuntu is APT, the Advanced Package Tool.
APT manages software packages on Debian and Debian-based distributions. Its tools retrieve package information from configured repositories, resolve dependencies, download packages, and coordinate installation, upgrades, and removal. Common commands include `apt install`, `apt update`, and `apt upgrade`. APT works with Debian’s `.deb` package format and uses lower-level package-handling machinery supplied by `dpkg`.
APT is better understood as a package-management system and collection of tools than as one single executable. The `apt`, `apt-get`, and `apt-cache` programs are prominent interfaces, while graphical tools such as Synaptic and software centers can act as front ends. A frequent confusion is treating APT and `dpkg` as interchangeable: `dpkg` handles individual local packages, whereas APT is designed to work with repositories and dependency relationships. Debian’s APT project dates to 1998, and Ubuntu inherited it as a core part of its Debian-based software ecosystem.