(via www.ubuntugeek.com)
The Advanced Packaging Tool, which has since been ported by Conectiva for use with rpm and has been adopted by some other distributions.The apt-get command is a powerful command-line tool used to work with Ubuntu’s Advanced Packaging Tool (APT) 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 Ubuntu system.