Navigation: Up to Applications 
Introduction
Ubuntu includes some powerful tools for managing software. Here are some of those tools and terminology:
The Basics
Installing Software - Find out how to install software.
Removing Software - Find out how to remove software.
Software Packaging Formats - Linux programs come packaged in a number of different ways. This is an overview of them.
Repositories increase the number of packages available to install on your system.
Switching To Ubuntu - Learn how Ubuntu compares to your current operating system.
On The Command Line
Using apt-get - How to use apt-get for package management.
Using Aptitude - How to use aptitude for package management.
Commandline Package Management Tools - Overview of the various command line package management tools.
Commandline Repository Management Tools - Managing Repositories from the command line.
Tasksel - Packages for various tasks can be installed using tasksel.
Configuration - Manage configuration files after a new Ubuntu release installation.
On Web-browser
http://appnr.com/ - Web-based package browser. (Currently works only in Firefox)
Further Topics
Installing .run Packages - Installing software packages (typically games) which use .run installer files.
Reinstalling same packages - How to install the same packages after a reinstallation of your system.
What are phased updates? - An explanation of what phased updates are, how they work in Ubuntu, and how they affect Ubuntu's users.