Configuration
Configuration of the Advanced Packaging Tool (APT) system repositories is stored in the /etc/apt/sources.list file and the /etc/apt/sources.list.d directory. An example of this file is referenced here, along with information on adding or removing repository references from the file.
You may edit the file to enable repositories or disable them. For example, to disable the requirement of inserting the Ubuntu CD-ROM whenever package operations occur, simply comment out the appropriate line for the CD-ROM, which appears at the top of the file:
# no more prompting for CD-ROM please # deb cdrom:[Ubuntu 13.04 _Raring Ringtail_ - Release i386 (20111013.1)]/ raring main restricted
Extra Repositories
In addition to the officially supported package repositories available for Ubuntu, there exist additional community-maintained repositories which add thousands more packages for potential installation. Two of the most popular are the Universe and Multiverse repositories. These repositories are not officially supported by Ubuntu, but because they are maintained by the community they generally provide packages which are safe for use with your Ubuntu computer.
Packages in the Multiverse repository often have licensing issues that prevent them from being distributed with a free operating system, and they may be illegal in your locality.
Be advised that neither the Universe or Multiverse repositories contain officially supported packages. In particular, there may not be security updates for these packages.
Many other package sources are available, sometimes even offering only one package, as in the case of package sources provided by the developer of a single application. You should always be very careful and cautious when using non-standard package sources, however. Research the source and packages carefully before performing any installation, as some package sources and their packages could render your system unstable or non-functional in some respects.
By default, the Universe and Multiverse repositories are enabled but if you would like to disable them edit /etc/apt/sources.list and comment the following lines:
deb http://archive.ubuntu.com/ubuntu raring universe multiverse deb-src http://archive.ubuntu.com/ubuntu raring universe multiverse deb http://us.archive.ubuntu.com/ubuntu/ raring universe deb-src http://us.archive.ubuntu.com/ubuntu/ raring universe deb http://us.archive.ubuntu.com/ubuntu/ raring-updates universe deb-src http://us.archive.ubuntu.com/ubuntu/ raring-updates universe deb http://us.archive.ubuntu.com/ubuntu/ raring multiverse deb-src http://us.archive.ubuntu.com/ubuntu/ raring multiverse deb http://us.archive.ubuntu.com/ubuntu/ raring-updates multiverse deb-src http://us.archive.ubuntu.com/ubuntu/ raring-updates multiverse deb http://security.ubuntu.com/ubuntu raring-security universe deb-src http://security.ubuntu.com/ubuntu raring-security universe deb http://security.ubuntu.com/ubuntu raring-security multiverse deb-src http://security.ubuntu.com/ubuntu raring-security multiverse