InstallingSoftware
There are several methods to graphically install software:
The Add Applications utility in the Applications menu
Synaptic Package Manager, which can be found in the System > Administration menu.
See CommandLinePackageManagement for how to manage software and packages.
APT
The main source of packages for Ubuntu is the Ubuntu software repository. You can use apt-get, synaptic, and 'Add applications' to query this repository and install software from it.
Adding repositories
Sometimes the default repositories are not enough. In these cases, extra repositories can be added to the APT database. You will often find information on repositories in the following format:
deb <url> <component-list>
You can add a repository in one of the following ways:
In Add Applications via Settings > Repositories > Add > Custom. Enter the supplied repository line here.
In Synaptic via the same menus as in Add Applications.
By manually editing the file /etc/apt/sources.list You will need superuser privileges for this, and please make a backup before editing. Just paste the repository line at the end of the file.
The page http://www.ubuntulinux.nl/source-o-matic can help you find extra repositories.
Add Applications
The add applications tool shows you a lot of desktop applications (such as games, office applications, e-mail and usenet clients etcetera). You can easily select software and let the system install it all for you.
Synaptic
The Synaptic application will show you all available packages in the repositories you have selected (Fortunately it has a search function too). Installing software is just as easy as with the previous tool.
Synaptic also has the possibility to download and instal updates to your system.