The apt-get command is a powerful command-line tool for performing such functions as installation of new software packages, upgrade of existing software packages, updating of the package list index, and even upgrading the entire linux distribution system on a computer.
The apt-get command is part of APT, or Advanced Packaging Tool. APT was originally designed as a front-end for dpkg to work with Debian's .deb packages. There is no single "apt" program; apt is a collection of tools. Two such programs are apt-get and apt-cache. This document focuses on apt-get.