What are Repositories?
There are literally thousands of Ubuntu programs available to meet the needs of Ubuntu users. Many of these programs are stored in software archives commonly referred to as repositories. Repositories make it very easy to install new software onto Ubuntu using an Internet connection, while also providing a high level of security, as each program available in the repositories is thoroughly tested and built specifically for each version of Ubuntu.
The Ubuntu software repositories are organized into four separate areas or "components", according to the level of support offered by Ubuntu and whether or not the program in question complies with Ubuntu's Free Software Philosophy.
The repository components are:
Main - Officially supported software.
Restricted - Supported software that is not available under a completely free license.
Universe - Community maintained software, i.e. not officially supported software.
Multiverse - Software that is not free.
Overview of the default Ubuntu software repositories
To organize the software, Ubuntu repositories are categorized into four groups: Main, Restricted, Universe, and Multiverse. The rationale used to determine which software goes into which category is based on the level of support that software development teams provide for a program and the level of compliance the program has to the Free Software Philosophy.
The standard Ubuntu Install CD contains some software from the Main and Restricted categories.
If your system is connected to the Internet, many more software programs are made available for installation. For example, the “Universe” and “Multiverse” repositories are only available over the Internet.
Please note: The Multiverse repository contains software which has been classified as non-free. This software may not be permitted in some jurisdictions. When installing each package from this repository, you should verify that the laws of your country permit you to use it. Also, this software may not include security updates.
The Update Manager automatically finds software updates for your computer when they are available. It regularly gathers information on potential updates from a number of online update sources.
If you click System → Administration → Software Sources and select the Updates tab, you will notice that four update sources are available. An explanation for each of these is provided below:
Important security updates: Updates which fix critical security flaws are made available through this source. It is recommended that all users leave this source enabled (it should be enabled by default).
Recommended updates: Updates which fix serious software problems (which are not security flaws) are made available through this source. Most users will want to leave this source enabled as common and annoying problems are often fixed with these updates.
Pre-released updates: Updates which are currently being tested before being released to everyone are provided through this update source. If you would like to help test new updates (and get fixes for problems more quickly), enable this source. Be aware that these updates may not yet be well tested; it is not recommended that you enable this source unless you are prepared to experience occasional problems.
Unsupported updates: When new versions of popular software are released they are sometimes “back-ported” to an older version of Ubuntu so that users can benefit from new features and fixes for problems. These backports are unsupported, may cause problems when installed and should only be used by people who are in desperate need of a new version of a software package which they know has been backported.