APT, or Advanced Package Tool, is the core software package management system used by Debian and its many popular derivatives, like Ubuntu. Operating primarily through command-line tools such as `apt` and `apt-get`, it automates the processes of installing, updating, and removing software, as well as upgrading the entire operating system. Its most powerful feature is its ability to handle dependency resolution, automatically identifying, retrieving, and installing any additional packages or libraries that a requested piece of software needs to function correctly. By managing software repositories (sources for packages), APT provides a robust and straightforward way to maintain a system's software, ensuring stability and easy access to a vast library of applications.