APT is short for Advanced Packaging Tool, the package manager for Debian-based distributions. It is also the most used tool for installing packages in Ubuntu. Questions about using any of the apt commands should use this tag.
APT uses dpkg (Debian package management system) as a backend. APT downloads the package list, keeps track of all packages installed, solves dependencies and removes unnecessary packages. It also allows you to query the cache, upgrade packages, manage packages from multiple repositories, etc.
APT consists of a generic
apt command, and several specialized subcommands:
apt: generic command line interface of apt, available from apt 1.0 (Ubuntu 14.04). Includes the functionality (in some cases limited) of several other utilities.
apt-get: the default package handling utility
apt-cache: query the apt cache
apt-key: manage the different GPG keys for repositories
apt-mark: to mark specific packages as automatically or manually installed
apt-config: to manage APT configuration.
apt-cdrom: manage CD-ROM entries
Questions about editing or configuring APT itself using the files in the directory
/etc/apt (e.g. pinning) and questions about other directories that are also related to APT, and the usage of the different APT APIs, calls and libraries, like python-apt should all be tagged with apt.
There are other front ends for most of the functions of the commands included in the apt package, and each of these should be referred to by their own tag. The most common examples are: aptitude and nala.