APT is an acronym for Advanced Packaging Tool, a package management system created by the Debian project. APT greatly simplifies the process of installing and removing software on Unix systems.
There is no
apt program per se; APT is a C++ library of functions that are used by several command line programs for dealing with packages, notably
apt-cache. There are also programs that provide a front end to APT, usually based on apt-get, like
aptitude with a ncurses text interface or
synaptic with a GTK+ graphical interface. There is a central repository of over 13000 apt packages used by apt-get and derived programs to download and install applications directly from the Internet, often hailed as one of Debian's best features.