APT, the Advanced Package Tool from the Debian project, is for managing packages by using a lot of separate tools to accomplish various tasks. In the past, users needed to know multiple command structures like apt-get, apt-cache, apt-config, and many more to utilize the full feature-set of APT.
APT was created originally to solve a lot of package management problems like putting an end to the dependency hell that so many people experienced in the early days of Linux-based operating systems.