When you install software on Ubuntu, the package manager automatically downloads the required software packages from a software repository, a location on the Internet which stores collections of packages ready to be downloaded.
There are thousands of programs available to install on Ubuntu. These programs are stored in software repositories and are made freely available for all Ubuntu users. This makes it very easy to install new programs, and it is also very secure because each program you install is built specially for Ubuntu and checked before it is allowed into the 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.
![[Warning]](../../libs/admon/warning.png)
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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 → → and select the Updates tab, you will notice that four update sources are available. An explanation for each of these is provided below:
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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).
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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.
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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.
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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.
