Acronym for Advanced Package Tools.
APT is a system that layers automatic package dependency resolution on top of an arbitrary PackageManagementTool. The original implementation uses Debian's Deb package format as a base, but with AptForRpm an implementation that supports RedHat's RPM format also exists.
The basic tool for apt is apt-get(8). Using it looks something like
[root@debian ~] # apt-get install foo Reading Package Lists... Done Building Dependency Tree... Done The following extra packages will be installed: bar baz The following packages will be upgraded bar foo baz The following packages will be REMOVED: quux 3 packages upgraded, 0 newly installed, 1 removed and 582 not upgraded. Need to get 12345kB of archives. After unpacking 1234kB of additional disk space will be used. Do you want to continue? [Y/n]
APT has calculated that in order to install the latest version of foo, you will need to upgrade to a newer bar and baz; quux is Deprecated by the new version of baz, so it will remove that package. It also calculated that with the old versions of the packages gone and the new ones updated and installed, an additional 1234KB disk space will be taken.